If y varies directly as x and y=12 when x=c what is y in terms of c when x=8

"y varies directly as x" means y = kx

you do not know what k is yet, other than that it is some constant number.

Now plug in the given values, y=12 when x=4, in the equation

12 = 4k

=> k = 3

Now rewrite the equation with the k value you just found

y = 3k

Substitute 18 in place of x

y = 3 (18)

y = 54