To solve, you would first take the equation y-5=-8, and add 5 on each sides. (To solve an equation, you need to isolate the variables. Adding 5 on both sides will cancel out the "- 5". in order to do this you must do it to both sides so it will still be even.) You should now have Y = -3. to check this, you can say "-3 - 5 =", and your answer will be -8.
