How do you solve #9x+9y=27# and #-9x+y=43# using substitution?

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Answer 1 · 2017-07-18T18:54:05

We will start with #9x+9y=27# and solve it for #x#

#9x+9y=27 iff x+y=3 iff x=3-y#

We then substitude this into #-9x+y=43#

So we get #-9x+y=43 iff -9(3-y)+y=43 iff #

#-27+9y+y=43 iff 10y=70 iff y=7#

We know know #y# so we can find #x#

#x=3-y=3-7=-4#

So the solution is #(-4,7)#