For the following system, if you isolated x in the second equation to use the substitution method, what expression would you substitute into the first equation?
3x + y = 8
−x − 2y = −10
a.-2y+10
b. 2y+10
c. 2y-10
d.-2y-10

The original equation we have is : −x − 2y = −10
We need to isolate the x, which means that we need to get x alone on one side of the equation preferably in a positive form.

So, going back to the original expression:
−x − 2y = −10
To make x positive, we will add x to both sides of the equation, thus, we get:
-2y = -10 + x
The next step is to eliminate the -10 from beside the x. To do this we will add 10 to both sides of the equation. When doing this we get:
x = -2y + 10

Based on this, the correct choice is A : -2y + 10