Question

If m∠4 = (3x)° and m∠8 = (x + 40)°, what is the measure of ∠4?

∠4 and ∠8 are vertical.

Answer 1

Use the fact that vertical angles are congruent.
m∠4=m∠8=(3x)°=(x+40)°
2x°=40°, x=20°
Hence m∠4=(3*20)°=60°

Answer 2

see if you have 2 lines crossing , the intersection has 4 angles, and the opposite ones are the same measure