in 3 years time (s+3 and d+3) sulivan is 3 times as old as daughter s+3=3(d+3)
6 years ago (s-6 and d-6) he was 6 times as old s-6=6(d-6)
expand
s+3=3d+9 s-6=6d-36 for 2nd equation, add 6 to both sides s=6d-30 sub 6d+30 for s in other equation 6d-30+3=3d+9 6d-27=3d+9 minus 3d both sides 3d-27=9 add 27 both sides 3d=36 divide both sides by 3 d=12
