first, find the slope. locate two corner points to make calculation easier and more accurate. Notice here (1,30) and (4,150) are right on the corner of a unit square.
m=(150-30)/(4-1)=40
the equation of the line is y=40x+b
next, find b by using one of the above two corner points. I'll use (1,30) here.
30=40*1+b
b=-10
It doesn't make sense that the starting number of stamps is negative. Let's just assume Katie owed her friend 10 stamps and let's ignore the negative number.
our equation is y=40x-10
when x is 25, y=40*25-10=990
