Simplify:
\(\left(\right.7y^{2}+3xy-9\left.\right)\)-\(\left(\right.2y^{2}+3xy-5\left.\right)\)

When subtracting polynomials the first step is to distribute the negative sign through the parentheses. This changes the sign on each term inside the parentheses:
\(7y^{2}+3xy-9-2y^{2}-3xy+5\)
Arrange like terms next to each other (optional):
\(7y^{2}-2y^{2}+3xy-3xy-9+5\)
Combine like terms to compute the answer:
\(5y^{2}-4\)