-11a^2+1ab+4b^2; P-Q would mean you rearrange the equation to where (-4a^2-2ab+9b^2)-(7a^2-3ab+5b^2). Don't forget to distribute the negative for Q since you plugged it into a variable.
-4a^2-2ab+9b^2-7a^2+3ab-5b^2
Add/Subtract like terms
-11a^2+1ab+4b^2
