Let's solve for #x#:
#-x-3y = 15#
#-x = 15+3y#
#x = -15-3y#
#* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *#
Now let's substitute that into the second equation
#2(-15-3y) + 7y = -36#
#-30 - 6y + 7y = -36#
#-6y + 7y = -6#
#y = -6#
#* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *#
Now we need to solve for #x#:
#x = -15-3(-6)#
#x = -15 + 18#
#x = 3#
#* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *#
We should still check our work:
Plug in #3# for #x# and #-6# for #y#
#-(3) - 3(-6) # should equal #15#
#-3 - (-18)#
#-3+18 = 15#
#15 = 15#, so we were right!!!
