Let x = the numerator of the fraction. Since the denominator is 12 bigger, it is x+12 and the fraction is
x/(x+12)
Add 2 to the numerator: x+2. Add 7 to the denominator: x+12+7 = x + 19, and it equals 1/2:
(x+2)/(x+19) = 1/2
Cross multiply: 2(x+2) = x + 19
Solve for x. Once you've found x, the fraction is x/(x+12)