Solve the equation for both -1 and 1 first.
50(1/2)^-1 = 100
50(1/2)^1 = 25
Now subtract the two answers and divide by 2 ( -1 to 1 is 2)
75/2 = 37.5
Because the answer got smaller from -1 to 1, the change would be negative.
The rate of change would be -37.5