Chapter 1 - Foundations for Algebra - 1-8 An Introduction to Equations - Practice and Problem-Solving Exercises - Page 57: 58

$y=-5$

In order to solve this algebraic expression, we must isolate the variable. A variable is the letter in the problem (for instance, in the equation x+2=10, x is the variable). Because addition and subtraction are inverse operations (they cancel each other out), we know we must add when a number is being subtracted from the variable and subtract when a number is being added to the variable. In the equation 9=4+(-y), we must get rid of the 4, which is on the same side as y, in order to solve. Thus, we subtract 4 on both sides of the equation to find that -y=5. We need y to be positive, so we divide by -1 on both sides of the equation to get that $y=-5$.