If f(x) varies directly with x, and f(x) = 45 when x=9, find the value of f(x) when x=3

Question

answer choices are
A. f(x)=8x
B. f(x)=4/3x
C. f(x)=6x
D. f(x)=3/4x

Answer 1

if f(x) varies directly with x, then
f(x) = kx, where k is a constant
given: if x=9 , f(x) = 49
so 49 = 9k
k = 45/9 = 5

f(x) = 5x

f(3) = 5(3) = 15

or

just use a ratio
9/3 = 45/f(x)
9f(x) = 135
f(x) = 135/9 = 15