When subtracting variables with exponents do you add or subtract the exponenets?

You cannot ad or subtract variables with different exponents: the exponents must be the same. The coefficients are added or subtracted and the exponent of the answer is the common exponent. (The rules are similar to those for the denominators of fractions.)

Thus 2x^2 + 5x^3 cannot be combined into a single term.

while 2x^2 + 5x^2 = (2+5)*x^2 = 7x^2