So,
We are trying to figure out when Grandpa Lopez's age was twice that of Dad.
Let x represent the number of years before/after when G. Lopez's age was twice that of Dad.
66 + x = 2(37 + x)
Distribute.
66 + x = 74 + 2x
Subtract x from both sides.
66 = 74 + x
Subtract 74 from both sides.
-8 = x
So 8 years ago, G. Lopez was twice as old as Dad. Let's check that.
66 - 8 = 58
37 = 8 = 29
29 * 2 = 58
58 = 58
It checks.