Yoo can someone tell me how to solve this: (x+5)(x^2+4x−8)

and this: (2h+3)(2h^2+3h+4)

i dont understand how to solve them, so a good explanation would be appreciated

How many liters of peroxide should be added to 12 liters of an 8% peroxide solution so that the resulting solution contains 16% peroxide?   original added new amount of peroxide 0.96 liters x 0.96 + x amount of solution 12 liters x 12 + x 0.18 liter 1.14 liters 1.92 liters 2.10 liters