Remember that the equation of the line in slope-intercept form is equal to
y=mx+b
where
m is the slope
b is the y-coordinate of the y-intercept
In this context
we have
C=mt+b
step 1
Find out the slope
we need two points
looking at the graph
we take (0,100) and (1,150)
so
m=(150-100)/(1-0)
m=50 --------> that means $50 per month (monthly fee to remain a member)
the y-intercept is the value of y when the value of x is zero
in this context, the y-intercept is the cost C when the value of t is zero
b=100 ------> that means a one-time fee to join
substitute
C=50t+100
and the slope is $50 per month (monthly fee to remain a member)