20+8y-9y-21 which expression is equivalent

Since you didn't provide expression to see which one is equivalent,, I will simply solve the expression you provided. If you need help finding out which expression option is equivalent after I solve this for you,, let me know and I would be more than happy to help you figure it out.
The first step for solving this expression is to calculate the difference between 20 and 21. We can start solving this by keeping the sign of the number with the larger absolute value and subtract the smaller absolute value from it. This will look like the following:
-(21 - 20)
Now subtract the numbers and add it back into the expression.
-1 + 8y - 9y
Next we need to collect the like terms with a y variable by subtracting their coefficients.
(8 - 9)y
Calculate the difference in the parenthesis.
-1y
Remember that when the term has a coefficient of -1,, the number doesn't have to be written but the sign must remain. 
-y
Lastly,, add this back into the expression to get your final answer.
-1 - y
Let me know if you have any further questions.
:)