How do you solve by substitution #4x-9y=-21# and #4x+3y=-9#?

1 Answer
Jun 8, 2015

#(A): 4x-9y=-21#
#(B): 4x+3y=-9#
We firstly obtain x or y from (A):
#4x=9y-21#

#x=9/4y-21/4#

And then we put this result at the place of x in (B).

#4(x)+3y=-9 -> 4(9/4y-21/4)+3y=-9#
#9y-21+3y=-9#
#12y=12#
#y=1#

It is not the end! Now that we found y we have to find x. We can basically substitute y=-5/2 in one of the two equations (for example #(A)# ).
#4x-9(y)=-21 -> 4x-9(1)=-21#
#4x=-21+9=-12#
#x=-3#

So the solution of the system is #(x,y)=(-3,1)#