How to do two step equlaity work problems

To solve the system of equations below, pedro isolated the variable y in the first equation and then substituted it into the second equation. what was the resulting equation? { 5y=10x {x^2+y^2=36

21 a stick 7 inches long is broken into two pieces, so that one piece is twice as long as the other one. how long are the two pieces?

Heather is a cashier. she can ring up 1212 customers in 99 minutes. at this rate, how many minutes does it take her to ring up 44 customers?