"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