Question

Solve the system using substitution. What is the x- or y-coordinate of the solution?
-2x - y = 3
3x + y = 2
A. y = 5
B. y = 7
C. y = 15/2
D. y = -13

Answer 1

Know how to do substitution?

Answer 2

To substitute is to put in place of. In general you solve for one of the variable, x or y, and then you put the right hand side of that solution in place of the variable on the OTHER equation:

In the case of:
\(ax+by=c\)
\(dx+ey=f\)

I can do something like this:
\(dx+ey=f\)
\(dx+ey-ey=f-ey\)
\(dx=f-ey\)
\(\dfrac{dx}{d}=\dfrac{f-ey}{d}\)
\(x=\dfrac{f-ey}{d}\)

Then I know what x is in terms of y. I can then replace that in the other equation:

\(ax+by=c\) becomes \(a\left(\dfrac{f-ey}{d}\right)+by=c\)

Then you can find y.

Answer 3

is it D?

Answer 4

Yah.