You can put the numbers where the variables are and do the arithmetic, or you can simplify the expression first. In the latter case, you have
(3a^(-2-(-1))b^(6-5)/2)^2 = (3a^(-1)b^1/2^2 = (3b/(2a))^2
= (3(-2)/(2(3)))^2 = (-1)^2 = 1
The value of the expression is 1.
