What does President Roosevelt say that is important that Americans NOT do in regards to things that they hear?

because in that time people did not hear clearly they heard something they did something

The vectors A = ⟨5, -4, 4⟩, B = ⟨-5, 0, -3⟩, and C = ⟨-4, 2, -5⟩ point to the points A, B, and C in the plane respectively.

Take any two of these vectors and compute their difference. This new vector will point in a direction parallel to the plane. (If it's unclear why, consider the triangle law of vector addition.) We'll need two such vectors. Here, I'll take

B - A = ⟨-5, 0, -3⟩ - ⟨5, -4, 4⟩ = ⟨-10, 4, -7⟩

and

C - A = ⟨-4, 2, -5⟩ - ⟨5, -4, 4⟩ = ⟨-9, 6, -9⟩

but you would get the same end result if you picked A - B and C - B, or A - C and B - C.

The cross product of any two vectors is perpendicular to both of those vectors. Both B - A and C - A point parallel to the same plane, so their cross product will be perpendicular to that plane.

(B - A) × (C - A) = ⟨-10, 4, -7⟩×⟨-9, 6, -9⟩ = ⟨6, -27, -24⟩

This answer is not unique, since any multiple of this vector will be perpendicular or normal to the given plane. For example, since each component is a multiple of 3, you can simplify the normal vector to ⟨2, -9, -8⟩.

In case you're unsure of how the cross product was computed: recall the definition of ×,

⟨1, 0, 0⟩×⟨0, 1, 0⟩ = ⟨0, 0, 1⟩

⟨0, 1, 0⟩×⟨0, 0, 1⟩ = ⟨1, 0, 0⟩

⟨0, 0, 1⟩×⟨1, 0, 0⟩ = ⟨0, 1, 0⟩

For any vector v, we have v × v = 0, the zero vector. Also, for any two vectors u and v, we have u × v = -(v × u).

Expand both vectors in terms of the unit vectors,

⟨-10, 4, -7⟩ = -10 ⟨1, 0, 0⟩ + 4 ⟨0, 1, 0⟩ - 7 ⟨0, 0, 1⟩

⟨-9, 6, -9⟩ = -9 ⟨1, 0, 0⟩ + 6 ⟨0, 1, 0⟩ - 9 ⟨0, 0, 1⟩

The cross product is distributive, so that

⟨-10, 4, -7⟩×⟨-9, 6, -9⟩

= (-10 ⟨1, 0, 0⟩ + 4 ⟨0, 1, 0⟩ - 7 ⟨0, 0, 1⟩) × (-9 ⟨1, 0, 0⟩ + 6 ⟨0, 1, 0⟩ - 9 ⟨0, 0, 1⟩)

= 90 ⟨1, 0, 0⟩×⟨1, 0, 0⟩ - 60 ⟨1, 0, 0⟩×⟨0, 1, 0⟩ + 90 ⟨1, 0, 0⟩×⟨0, 0, 1⟩

… - 36 ⟨0, 1, 0⟩×⟨1, 0, 0⟩ + 24 ⟨0, 1, 0⟩×⟨0, 1, 0⟩ - 36 ⟨0, 1, 0⟩×⟨0, 0, 1⟩

… + 63 ⟨0, 0, 1⟩×⟨1, 0, 0⟩ - 42 ⟨0, 0, 1⟩×⟨0, 1, 0⟩ + 63 ⟨0, 0, 1⟩×⟨0, 0, 1⟩

= -60 ⟨0, 0, 1⟩ - 90 ⟨0, 1, 0⟩ + 36 ⟨0, 0, 1⟩ - 36 ⟨1, 0, 0⟩ + 63 ⟨0, 1, 0⟩ + 42 ⟨1, 0, 0⟩

= -24 ⟨0, 0, 1⟩ - 27 ⟨0, 1, 0⟩ + 6 ⟨1, 0, 0⟩

= ⟨6, -27, -24⟩