Explain
(picture below)

i have part a however idk how to work out part b how would i write the equation

The generic equation of the line is:
 y-yo = m (x-xo)
 We look for the slope:
 m = (y2-y1) / (x2-x1)
 m = (18-15) / (9-7)
 m = (3) / (2)
 m = 3/2
 We write the equation choosing any point:
 y-15 = (3/2) (x-7)
 we rewrite the equation:
 y-15 = (3/2) x-10.5
 y = (3/2) x-10.5 + 15
 y = (3/2) x + 4.5
 Then, for x = 13 we have:
 y = (3/2) * (13) +4.5
 y = 24
  
 The equation is:
 y = (3/2) x + 4.5
 The number of matches won for 13 months is:
 y = 24