7+3x=5x+13
Subtract 3x from both sides of the equation: 7 + 3x - 3x = 5x + 13 - 3x
7 = 2x + 13
Subtract 13 from both sides of the equation: 7 - 13 = 2x + 13 - 13
-6 = 2x
Divide both sides of the equation by 2: -6 / 2 = 2x / 2
-3 = x, or more conventionally, x = -3
Put that value back into the original equation to verify:
7+3x=5x+13
7 + (3 * -3) = (5 * -3) + 13
7 - 9 = -15 + 13
-2 = -2