So, what you would do is multiply:
12(1) - 12(x) + 7 = 100
12 - 12x + 7 = 100
Now put the x's on one side and non-x's on the other side of the equal sign.
12x = 100 - 7 - 12
simplify.
12x = 81
bring 12 as the denominator and 81 as numerator
x = 81/12