Simplify the expression √x2 - 25 as much as possible after substituting 5csc(θ) for x.

I think you mean

√(x^2 - 25)

then
√(25csc^2 Ø - 25)
= 5√(csc^2 Ø - 1)
= 5√(tan^2 Ø )
= 5tanØ