On the left side of the equation, x is being multiplied by 0, which makes the left side equal to zero.
0 = (10 * 50) - (10 * 4x)
0 = 500 - 40x
Add 40x to both sides
0 + 40x = 500 - 40x + 40x
40x = 500
Divide both sides by zero
40x/40 = 500/40
x = 12.5