This is a textbook problem...

f:R->R

f(x) = x^2

(i)Find f^-1(25).

(ii)Find f^-1([4, 25])

The first one is easy but how do I do the 2nd one? I've never encountered something like this.

I suppose

Let f^-1([4,25])=x

or, f(x)=[4,25]

or, x^2=[4,25]

...and then?