Answer:
I think the word you are looking for is the in fact the word idea itself.
Explanation:
He took exactly four years
Move all terms containing x to the left, all other terms to the right.
Add ‘-28’ to each side of the equation.
28+-28+2x=40+-28
Combine like terms: 28+-28=0
0+2x=40 + 28
2x=40+-28
Combine like terms: 40+-28=12
2x=12
Divide each side by ‘2’
X=6
Simplifying
X=6