By using the Distributive Property, you would multiply 3 into the parenthesis (3 · x and 3 · -4)
3(x - 4) = 24
3x - 12 = 24
Then, you would solve like any normal one-step equation.
3x - 12 = 24
+12 +12
__________
3x 36
__ = __
3 3
x = 12, which is your final solution.
If you need any further explanations, please ask me :)