How do I find the percent of coverage for these problems?

How do i work this problem out using the equation a=pe^rt

Apublic library wants to place 4 magazines and 9 books on each display shelf. the expression 4s+9s represents the total number of items that will be displayed on s shelves. simplify the expression

If f(x)=-x^2+6x-1 and g(x)=3x^2-4x-1,find(f+g)(x)