Finally:
You can solve the others the same way. I hope it has been helpful.
a.) (49)^(-3/2) is the same as writing 1 / [(49)^(3/2)]
The 3/2 part is the same as the cube root of 49^2.
b.) (27/8)^(1/3) = (8/27)^(1/3)
c.) The exponent part takes precendence over the negative, which counts as a multiplication by -1. Therefore - [25^(1/2)], which just means to take the square root of 25 and multiply by -1. Answer is -5.