Y+9=13

Jbow4 is correct, but in case you want further explanation:

Y + 9 = 13

Your overall goal is to get one variable isolated (which is defined as: put the variable, in this case y, on one side of the "=" by itself.)

This is a balance beam; whatever you do to one side of the equation, you must do to the other.

If we subtract 9 from both sides, we can easily get the answer:

Y + 9 - 9 = 13 - 9

Obviously 9 - 9 = 0, so we're left with Y + 0 or just Y on the left side. On the right side 13 - 9 = 4:

Y = 4

This is our answer.

to find y, you need to  find the difference between 13 and 9. To do this, you will need to subtract 9 from 13, giving you 4.