Find f(x)if f(3) = 0 and f(−4) = 0.

not enough information.

is f(x) linear ?

we must assume that, since you only have 2 points, namely, (3,0) and (-4,0)

slope = 0/-7 = 0
so we have a horizontal line and
f(x) = 0
f(x) is the x-axis