I must assume that you meant 14/20 = 17 / (x+12). If so, it's important to use parentheses to indicate that you are dividing 17 by (x+12).
Then your equation of proportions (ratios) is
14 17
------ = -----------------
20 x+12
Let's cross-multiply: 14(x+12) = 340
Divide both sides by 14 to isolate (x+12): x+12 = 340/14 = 24.29
Subtracting 12 from both sides, we get x = 12.29
You should check this answer by subst. it into the original equation of ratios.