The product rule says that to multiply factors with like bases you add the exponents. You may have done that with the variable factors, but you also added the exponent of the constant 7 to the other exponents. The numbers 7 and y are unlike bases. Also, the factor 7 should only have an exponent of 1, but since it is inside the parentheses, the exponent 10 is being applied to it too. The correct answer is .