"a is 4 more than b" ⇒ a = b +4
"a is 1 lesss than b" ⇒ a = b - 1
The first pair of equations is the appropriate choice.
2x + 7 = 21
first you subtract the 7 from the 7 and 21 which will leave you with 2x=14
then see what multiples with 2 to get 14 which is 7!