Let's use the slope intercept form of a line which is:
y=mx+b where m=slope and b=y-intercept.
Mathematically m=(y2-y1)/(x2-x1) and the y-intercept is just the value of y when x is equal to zero, where the line intercepts the y axis...
First you want to find the slope by finding two clearly identifiable points on the graph...I'll pick (4,4) and (0,1), now we can calculate the slope or "m"...
m=(4-1)/(4-0)=3/4 so now we can say our line is:
y=0.75x+b, using point (4,4) we can now solve for b.
