Solve the porportions. First, cross multiply
7/(x - 4) = 42/12
(7/(x-4))(x-4)(12) = (42/12)(12)(x-4)
7(12) = 42(x-4)
Simplify. (remember to distribute the 42 to both x and -4
84 = 42x - 168
Isolate the x. First add 168 to both sides
84 (+168) = 42x - 168 (+168)
42x = 84 + 168
42x = 252
Finally, isolate the x by dividing 42 from both sides
42x/42 = 252/42
x = 252/42
x = 6
6 is your answer
hope this helps