How close is the t-value to the corresponding z-value (at the bottom of the column for d.f.= ∞)?

28 units

Explanation:

This is an isosceles triangle.  We can tell this by the fact that the base angles, ∠C and ∠B, are marked congruent.

In an isosceles triangle, the two sides adjacent to the two congruent angles are also congruent.  This means that AC ≅ AB; it also gives us the equation

2x+7 = 4x-7

We do not want a variable on both sides, so we can start by subtracting 2x from each side:

2x+7-2x = 4x-7-2x

7 = 2x-7

Add 7 to both sides to cancel it on the right:

7+7 = 2x-7+7

14 = 2x

Divide both sides by 2:

14/2 = 2x/2

7 = x

The length of BC is 4x units; this means it is 4(7) = 28 units.