You need to find opposite terms when using the elimination method to solve a system of equations so that way you can find out the y or the x after you have crossed out the fraternal twins. You have to get rid of either the y or x in order to work it out and find the y or x value.
Well you need to find the opposite term so that you can plug it back in to the original question to solve for the variable you want with no other variables. Hope This Helps!
19500 times because 5 times 26 = 130 (how many minutes) and now just multiply how many minutes (130) times how many heart beats per minute (150) to get 19500