Solve the system of equations {3x-y=6 { 6x+y=21 explain the steps on how to do this

I would set the equations on top of each other and subtract

6x+y=21
- 3x-y=6

3x=15

Then solve for x
X=5

To solve for y, plug 5 in for x for one of the equations

The easiest way to solve this is by elimination. 

3x - y = 6 
6x + y = 21

Since you have a negative and positive y with the same coefficients (1), they cancel, and you add the other terms so it would look like:

9x = 27 

Then solving for x leaves you with x = 3 

Then you take the x value of three and plug it into to either of the equations, so 

3(3) - y = 6

9 - y = 6 

subtracting 9 

-y = -3 

then dividing by -1 

y = 3 

so the solution is x = 3 y = 3 or (3,3) 

The answer of the above is

You can find the area of a rectangle by multiplying the length and height. so multiply 1/2*7/4(which is the simpler version of 1 3/4) and you get 7/8 as your answer.