Please help me find angle B

So, first of all, all three angles in a triangle add up to 180°.

On the diagram, angle A is marked as 40°, and angle C is marked as 90°.

You can subtract those numbers from 180° to get the remaining angle.

180-40-90=50

Angle B = 50°

-2x + 7 = 7

subtract 7 from both sides.

-2x = 0

divide both sides by -2.

x = 0

the value of x is equal to 0.

How am i goonna know explain