f(x) = ax
passes through the point (2, 1/25), meaning f(2) = 1/25
Then x=2.
f(2) = a2 = 1/25
a2 = (1/5)2
a = 1/5
If a is not equal to zero, it means that the y-intercept is 1 because any number(except zero) raise to zero is one.
Therefore your exponential function is:
f(x) = (1/5)x