How do you solve the system of equations y=-9x-21y=−9x−21 and y=5x+7y=5x+7?

1 Answer
flyingmadpakke
Jun 10, 2017

x = -2x=−2

Explanation:

Look at the equations:
y = -9x-21y=−9x−21
and
y = 5x+7y=5x+7

Because both equations equal (y), they both equal the same.

If an equations says "y = -9x-21y=−9x−21" , that means we can replace (y) with -9x-21−9x−21.

Therefore when i take the equation: y = 5x+7y=5x+7 , we can replace (y) to get:
-9x-21 = 5x+7−9x−21=5x+7

Now isolate (x):
-9x-21 = 5x+7 iff -21 = 14x +7 iff -28 = 14x−9x−21=5x+7⇔−21=14x+7⇔−28=14x
iff x = -2⇔x=−2

Alternatively, you could solve it with graphs. Get a coordinatsystem and look at where the graphs intersect. They will intersect where graph 1 is equal to graph 2, which is essentially the same as what we just did :)

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