Write an equation in slope intercept form for the line with slope three and Y intercept -1

The equation in slope intercept form for the line with slope 3 and Y intercept -1 is y = 3x -1 .

Let r = small tier and s be medium tier.  
Then 3r + s = 100 (1) 
And 3r + 2s = 140 (2) 
Subtracting (2) from (1) we have 
 s = 40. 
 Substituting value for r into (1) we have
 3r + 40 = 100.
 3r = 100 - 40 = 60
 3r = 60
 r = 20.
 If U = 3r + s = 100 = 3r + s - 100
 Then du/dr = 3 and du/ds = 1
 Also, du/dr = 3 and du/ds = 2