you have to apply this formula (a+b)(c-d)=ac-ad+bc-bd
-6a^3*b*5a^2+6a^3b*2ab+6a^3b*b+2ab^2*5a^2-2ab^2*2ab-2ab^2*b=
-30a^5*b+12a^4b^2+6a^3b^2+10a^3b^2-4a^2b^3-2ab^3
i thimk this might be the answer to question 3 (iii)
I'm sorry I don't know how to solve the first question
Only 20%of earths water is useable