SOLUTION: Please help. Solve the system by substitution. 3x - y = -7 x + y = -9 Thanks JMS

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Question 115292: Please help.
Solve the system by substitution.
3x - y = -7
x + y = -9
Thanks
JMS
Answer by solver91311(12126) About Me  (Show Source):
You can put this solution on YOUR website!
Solution by substitution means that you need to solve one of the equations in terms of one of the variables and then use the expression for that variable in place of the variable in the second equation -- in other words, substitute it.

3x-y=-7. Solve it for y
-y=-3x-7
y=3x%2B7

Get the second equation
x%2By=-9

And substitute the value we found for y above (y=3x%2B7), in the place of y in the second equation.
x%2B%283x%2B7%29=-9

4x%2B7=-9
4x=-16
x=-4

Now that we have a value for x, we can substitute that back into either equation to find the value of y.

x%2By=-9
-4%2By=-9
y=-5

Does this check?
3%28-4%29-%28-5%29=-7?
-12%2B5=-7, check

-4%2B%28-5%29=-9, check

If you graphed these lines, you would see that the two lines intersect at the point (-4,-5).

graph%28400%2C400%2C-12%2C2%2C-12%2C2%2C3x%2B7%2C-x-9%29