Use the "^" character to indicate an exponent (just like on a calculator).
Put multiple-term numerators and denominators in parentheses. Assuming "x2" and "n2" mean and , what you posted meant:
I suspect what you intended was:
Did you really mean to have three variables (x, n and b)?? I suspect not.
Please re-post your problem making sure the problem is correct and using the "^" and parentheses to make the expressions unambiguous.