(-5/7)*(y-28)-((1/21)*y) = 0
(-5/7)*(y-28)+(-1/21)*y = 0
-5/7*(y-28)-y/21 = 0
(-5/7*21*(y-28))/21-y/21 = 0
-5/7*21*(y-28)-y = 0
420-16*y = 0
(420-16*y)/21 = 0
(420-16*y)/21 = 0 // * 21
420-16*y = 0
420-16*y = 0 // - 420
-16*y = -420 // : -16
y = -420/(-16)
y = 105/4
