you could see the pattern
it cost 3.8 cent to print 1 dollar,
It cost 2 x 3.8 cents to print 2 dollar
it cost 3 x 3.8 cents to print 3 dollar
First equation
f(x) = 3.8 x
Same with second equation,
f(x) = 15x
for equation 3, it would be :
y = 1.50 x - 0.8x
Hope this helps