We have f(x) = ax^r

If f(2) = 1, that means that a*2^r = 1

If f(128) = 5, that means that a*128^r = 5

We currently have two equations with two variables each.

\(a*2^r = 1\)

\(a*128^r=5\)

now, it's just a system of equations, solving for a and r. Can you take it from here?