Remember:
In mathematics there are often many acceptable solutions to a
problem. These answers show you "possible"
solutions. While the final answers are always the same, the methods of
arriving at the answers may differ.

1.

Solution
by Addition or Subtraction

Solution
by Substitution

x + y = 1 x + 3y = 9

x + y = 1 subtract -x -3y = -9

-2y = -8

y = 4
answer for y

x + 4 = 1 substitute to get
answer for x x = -3

check:

x + y = 1 x + 3y = 9

y = 1 - x solve first
equation for "y="

x + 3(1 - x) = 9
substitute into the second

x + 3 - 3x = 9
-2x + 3 = 9
-2x = 6 x = -3 answer
for x

x + y = 1 substitute
to get answer for y
-3 + y = 1 y = 4