1. (2y+1)*(5y-6)=10y^2+5y-12y-6=10y^2-7y-6

2. (3a-b)*(2a-7b)=6a^2-2ab-21ab+7b^2=6a^2-23ab+7b^2

3. (a-4b)*(a^2+3ab+6b^2)=a^3+3b*a^2+6a*b^2-4b*a^2-3a*b^2-24b^3=a^3-b*a^2+3a*b^2-24b^3

4. a(4a-5)(2a+3)=(4a^2-5a)(2a+3)=8a^3+12a^2-10a^2-15a=8a^3+2a^2-15a