In common with all non-zero numbers, #-25# has two square roots.
The square root represented by the symbols #sqrt(-25)# is the principal square root #i sqrt(25) = 5i#. The other square root is #-sqrt(-25) = -i sqrt(25) = -5i#.
When #a, b >= 0# then #sqrt(ab) = sqrt(a)sqrt(b)#, but that fails if both #a < 0# and #b < 0# as in this example: