If QR=2x-5 and RS=12 find QS

Me! last question, and i'm stuck! write the algebraic expression that models the word phrase. the product of 2 divided by the number h and 8 more than the number k

If it 8 inch candle burns at 7/10 inch per hour and a 6 inch candle burns at 1/5 inch per hour after how many hours would both candles be the same height

Find the value of x. also give me an explanation so i can complete it to other questions as well, : )