If f(x) varies directly with x, and f(x) = 18 when x = 2, find the value of f(x) when x = 6.
a.
6
b.
18
c.
36
d.
54

If f(x) varies directly with x, that mean there exists some unknown number k such that f(x) = kx.

So we were given that f(x) = 18 when x = 2. So plugging in these values will help you find the value of k.

18 = 2k

k = 9

So we know that k=9, therefore f(x) = 9x.

Now, at x=6, you have f(6) = 9·6 = 54

Therefore answer is D.