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

1 Answer
flyingmadpakke
Jun 10, 2017

#x = -2#

Explanation:

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

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

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

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

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