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.

 Since we already know s = 4




:)