I don't understand how to do this. Can someone explain the steps?​

S=3(3+2r+t)

2 Substitute
s
=
3
(
3
+
2
r
+
t
)
s=3(3+2r+t) into
5
r
+
5
s
−
5
t
=
20
5r+5s−5t=20.
35
r
+
45
+
10
t
=
20
35r+45+10t=20

3 Substitute
s
=
3
(
3
+
2
r
+
t
)
s=3(3+2r+t) into
3
r
−
s
+
4
t
=
−
5
3r−s+4t=−5.
−
3
r
−
9
+
t
=
−
5
−3r−9+t=−5

4 Solve for
t
t in
−
3
r
−
9
+
t
=
−
5
−3r−9+t=−5.
t
=
3
r
+
4
t=3r+4

5 Substitute
t
=
3
r
+
4
t=3r+4 into
s
=
3
(
3
+
2
r
+
t
)
s=3(3+2r+t).
s
=
3
(
7
+
5
r
)
s=3(7+5r)

6 Substitute
t
=
3
r
+
4
t=3r+4 into
35
r
+
45
+
10
t
=
20
35r+45+10t=20.
65
r
+
85
=
20
65r+85=20

7 Solve for
r
r in
65
r
+
85
=
20
65r+85=20.
r
=
−
1
r=−1

8 Substitute
r
=
−
1
r=−1 into
s
=
3
(
7
+
5
r
)
s=3(7+5r).
s
=
6
s=6

9 Substitute
r
=
−
1
r=−1 into
t
=
3
r
+
4
t=3r+4.
t
=
1
t=1

10 Therefore,
r
=
−
1
s
=
6
t
=
1

​

r=−1
s=6
t=1
​

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