Which is the best approximation for the measure of angle abc?
side ab = 20
side bc = 10.5
angle c is the right triangle
angle b = x

We can find the approximate measurement of angle ABC  and it is equal to 180°. The summation of all angles in a triangle is 180°.
Solving for x or angle B, we need to used Pythagorean theorem and SOH CAH TOA.
Solving for angle A:
sinA = 10.5/20
A = 31.67°

Solving for angle B:
180° = ∠C + ∠B + ∠A
180° = 90° + ∠B + 31.67
∠B = 58.33°

The x value is 58.33°.

The expression of this would be 3.75x+y= 23