we know that
The equation of the line in slope intercept form is equal to
y=mx+b
where
m is the slope or unit rate
b is the y-intercept or initial value
In this problem we have
m=$5 per class
b=$25
so
y=5x+25
where
x is the number of class
y is the total charge
therefore
the answer is
the rate of change is $5 per class