You have one unknown y and 2 numbers 12 and 8
We do not know his age now, then
Let his age now is y
From 8 years ago he was 12 years old
That means If we subtract 8 from his age now the answer should be 12
Then subtract 8 from his age now y to get 12
The equation is y - 8 = 12
To solve the equation add 8 to both sides, to move 8 from the left side to the right side
y - 8 + 8 = 12 + 8
y = 20
His age now is 20