2n

The sum of n and twice n means we add

n + 2n

The word

*is*means equal to, so we set that expression above equal to 12

n + 2n = 12

Combine like terms:

3n = 12

Divide each side of the equation by 3 to isolate n

n = 4

Check our work

Twice n is 2*4 = 8

Add that to n = 4

8 + 4

12