Question 180219
5r - 4s - 3t = 3
r = 3s + 1 
t = s + r
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You want to have one equation with one variable.
Find a variable that is associated with two others, like "s"
r = 3s+1
t =s + r = s+ 3s+1 = 4s+1
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Now substitute for "r" and for "t" in the 1st equation to get:

5(3s+1) - 4s - 3(4s+1) = 3

Solve for "s".

15s + 5 - 4s - 12s - 3 = 3

-s +2 = 3
-s = 1
s = -1
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Substitute to solve for r and for t:
r = 3s+1 = 3(-1) + 1 = -2
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t = 4s+1 = 4(-1) + 1 = -3
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Final Answer: 
s = -1
r = -2
t = -3
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Cheers,
Stan H.