Question 849620
Start out by using the Distributive property. 

2(a+1)=18 becomes

2a+2=18

Subtract 2 from each side.

2a = 16

Divide each side by 2

a = 8
Plug in the value of a into the original equation to see if it checks out.

2(8+1) = 18

2(9) = 18

18 = 18. Everything checks out so a = 8 is correct.