How do you write #y*y# as an exponential form?
Whenever you have a number times itself x times, you put the number being repeated on the bottom as the base, and the number of times it is repeated is put on the top right corner as the exponent.
Here's an example -- 2^4. 2 is the number being multiplied, so we compute that number as the base. The number of times 2 is being repeated is four. Thus the result 222*2, in other words, 2^4.
Back to the question. Number being repeated? y. Number of times y is being repeated? 2. Thus the answer is y^2.