36
how many years ago (x years) was sandra (36-x) 3 times as old as son (at that time, 14-x)
sandra's age x years ago is 3 times son's age x years ago
36-x=3(14-x)
solve
distribute
36-x=42-3x
add 3x both sides
36+2x=42
minus 36
2x=6
divide 2
x=3
3 years ago
(36-3=33, 14-3=11, 11*3=33)