For stuff like this you have to simplify the equation and solve for x.
But since its multiple choice you can just plug in each answer until it makes the equation true.
So here are the equations with n substituted in:
8(8+1)+3=45
7(7+1)+3=45
6(6+1)+3=45
5(5+1)+3=45
Now simplify:
8(8+1)+3=45 becomes 72+3 = 45
6(6+1)+3=45 becomes 42+3=45
5(5+1)+3=45 becomes 30+3=45
Its easiest to start with the highest option given and then go down. Because 8 is already way to high you could skip doing the math for 7 and go straight to 5 and 6.
42+3=45 becomes 45=45
30+3=45 becomes 33=45
So 6 makes this statement true.
After awhile its easy to gauge what numbers will come out to be, so you don't have to do the math for each answer.