Verified answer

what you are expected to do is factor the smallest possible root...

√(125) = √(5*5*5)=5√5

√(20)=√(4*5)=2√(5)

√(45)=√(9*5)=3√(5)

so your expression becomes

4*5√5+3*2√5-7*3√5

=√5(20+6-21)

=5√5