Since you did not provide the expressions to find out which one is equivalent to 20 - 8y - 8y - 21,, I will simply solve the expression you provided. From there you can figure out which expression you were given was correct or you can provide me with your option choices and I can help you figure it out.
The first step for solving this expression is to calculate the difference between 20 and -21.
-1 + 8y - 9y
Now collect the like terms for y by subtracting their coefficients.
(8 - 9)y
Subtract the numbers in the parenthesis.
-1y
When a term has a coefficient of -1,, the number doesn't have to written but the sign needs to remain. Knowing this,, our final answer will be the following:
-1 - y
Let me know if you have any further questions.
:)