of course, but this problem is solvable without any understanding of complex numbers, and 10*i^2 is a really clunky, multi-operation expression whereas -10 is just an integer.

simplifying one's answers is standard practice and any grader who received the answer in the OP would be obligated to point out that while technically correct, they're missing the basic fact that the answer is -10.

The Rube Goldberg comment is apt as the solution is absurdly complicated and overengineered for the task it performs.