Could someone explain this to me?

Explanation:

Let's focus on the root x = 2i. Square both sides to get rid of the imaginary component and then get everything to one side.

x = 2i

x^2 = (2i)^2

x^2 = 4i^2

x^2 = -4

x^2+4 = 0

You should find that the root x = -2i leads to x^2+4 = 0 as well.

So the equation x^2+4 = 0 produces the complex roots x = 2i and x = -2i. This tells us that x^2+4 is a factor of the answer. The other factor is x-5 due to the real number root x = 5.

We'll multiply those two factors mentioned:

(x-5)(x^2+4)

w(x^2+4) let w = x-5

wx^2 + 4w

x^2(w) + 4(w)

x^2(x-5) + 4(x-5) plug in w = x-5

x^3 - 5x^2 + 4x - 20

The last step is to triple each term so that we go from the leading term of x^3 to 3x^3. This will give us a lead coefficient of 3.

So we go from this

x^3 - 5x^2 + 4x - 20

to this

3x^3 - 15x^2 + 12x - 60

after tripling everything. This expression has the same roots as the previous one. Scaling the function only stretches it vertically, but the roots aren't changed.

Step-by-step explanation: