Question 35917
The two equations:

3x-y=21

2x+y=4

Step 1.

Solve for one of the varriables first.Simplfy the equation. 

2x+y=4

subtract 2x from both sides

y=-2x+4

You now know what "y" is equal to.

Next you will plug what "y" equals into the first equation( 3x-y=21)

So the new equation would look like this:

3x+2x-4=21

5x=25

x=5

Now find out what "y" equals.

y=-2x+4

y=-10+4

y=-6

The Check Step.

You now know that y=-6 and x=5

You will plug the numbers back into the first or second equation.

3x-y=21

3(5)-(-6)=21

15+6=21

21=21