Question 709394
The secant line?  You have two points for this line, so you use them as given to find the line.  Two points determine a line.  You have the function which gives the y coordinate in the pair of points.  Evaluate F(5) and F(7).  


The two points are:  (5,{{{4*5-8}}}) and (7,{{{4*7-8}}}).  Those coordinates computed, the points are (5,12) and (7,20).  


More?
m=(20-12)/(7-5)
m=4.

y-intercept?
y=mx+b, b=y-mx, use either point:
b=20-4*7, b=-18,
Line wanted is {{{highlight(y=4x-18)}}}, the secant line that contains (5, f(5)) and (7, f(7)).