Question 764996
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3*x - 5*y = s
This line passes through (3,1). That means the above equation is valid for
x = 3 and y = 1.

Substituting, 3*3 - 5*1 = s. 
i.e. s = 4.


This line also passes through (5,r). That means the above equation is valid for
x = 5 and y = r.

{{{3*5 - 5*r = s = 4}}} Since we already know s = 4
{{{15 - 5*r = 4}}}
{{{5*r = 11}}}
{{{r = 11/5}}}

:)
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