If a^x=b^y=(ab)^z prove that xy=z(x+y)

Which expression is equivalent to (3^2)^-2 a.-81 b.-12 c.1/81 d.1/12

Can someone me and do the problem plz so i can understand it more better

George has seven boxes of five pencils. giulio three boxes of eight pencils. one of the boys arranges all his pencils equally into two groups. was this george or giulio how many are in each group explain