Step 1) Because the first equation is already solved for y
we can substitute
3
x
+
8
for
y
in the second equation and solve for
x
:

8
x
+
4
y
=
12
becomes:

8
x
+
4
(
3
x
+
8
)
=
12

8
x
+
(
4
⋅
3
x
)
+
(
4
⋅
8
)
=
12

8
x
+
12
x
+
32
=
12

(
8
+
12
)
x
+
32
=
12

20
x
+
32
=
12

20
x
+
32
−
32
=
12
−
32

20
x
+
0
=
−
20

20
x
=
−
20

20
x
20
=
−
20
20

20
x
20
=
−
1

x
=
−
1

Step 2) Substitute
−
1
for
x
in the first equation and calculate
y
:

y
=
3
x
+
8
becomes:

y
=
(
3
⋅
−
1
)
+
8

y
=
−
3
+
8

y
=
5

The solution is:
x
=
−
1
and
y
=
5
or
(
−
1
,
5
)