math problem check

Hi,

I have this problem from my workbook in math, according to the calculator it says it equals to -1, am i missing anything? thanks!

x = (1/.817^2-1/.31151811^2)*(.817+.31151811^2/.817)/((.817/.31151811^2)-(.31151811^2/.817^3))

it was originally written as this picture:


while b = 0.817
and c:

thank you very much!