Seven boys were to have their pictures taken three at a time. the photographer had

Any one of the 7 boys could be in the first chair.  For each of those . . .

Any one of the remaining 6 boys could be in the 2nd chair.  For each of those . . .
Any one of the remaining 5 boys could be in the third chair.

Total possible ways to set up 3 out of 7 boys for a picture = (7x 6 x 5) = 210 .

Add both equations , the y’s will go . You will have 2x=14 so the x is 7 , replace the x with 7 in any of the equations you will get 7-3y=-2
Y=-3