Solve the system
9x = 27-9y
20x=71-9y
Let's use the "elimination through addition/subtraction method.
Multiply the first equation by -1 so as to obtain +9y:
-9x = -27 + 9y
20x= 71- 9y
--------------------
Add these 2 equations together:
-9x = -27 + 9y
20x= 71- 9y
--------------------
11x = 44
Solve this for x: x = 44/11 = 4.
Now find y by subst. 4 for x in either of the original equations.
Using the second equation:
20x=71-9y
20(4) = 71 - 9y
80-71 = -9y
9 = -9y. Then y = -1.
The solution set is (4,-1).
