What expression is equivalent to 20+8y-9y-21

The first step for solving this expression is to calculate the difference between 20 and -21. You will start this process by keeping the sign of the number with the larger absolute value and then subtract the smaller absolute value from the larger one. This will look like the following:
-(21 - 20)
Subtract the number in the parenthesis and add it back into our original expression.
-1 + 8y - 9y
Collect the like terms with y by subtracting their coefficients.
(8 - 9)y
Calculate the difference in the parenthesis.
-1y
Remember that when a term has a coefficient of -1,, the number doesn't have to be written but the sign needs to remain. Add this answer into your expression to find your final answer.
-1 - y
Let me know if you have any further questions.
:)

2160ft^3 i think you could use a formala to you out with this in the future