Think of two numbers that multiply to 125. One way is 25*5 = 125. Note how one of the factors (25) is a perfect square. We'll use this to help simplify sqrt(-125)
2*sqrt(-125) = 2*sqrt(-1*25*5)
2*sqrt(-125) = 2*sqrt(-1)*sqrt(25)*sqrt(5)
2*sqrt(-125) = 2*i*5*sqrt(5)
2*sqrt(-125) = 2*5*i*sqrt(5)
2*sqrt(-125) = 10i*sqrt(5)
So you have the correct answer. Nice job.