Question 444986
The slope of a linear function measure {{{the}}}{{{ rate}}} of increase per unit. For straight lines, the rate of change (slope) is {{{constant}}} (always the same).  

so, you have two points on your graph and you need to calculate a slope {{{m}}}:

(3,40) and (7, 210)



slope {{{m=(y2-y1)/(x2-x1)}}}

{{{m=(210-40)/(7-3)}}}

{{{m=170/4}}}

{{{m=42.5}}}

Intercept is found from equation {{{mx1+b=y1}}}, or {{{3m+b=40}}}. 
From that,
intercept {{{b}}} is {{{b=y1-3m=40-3*42.5=40-127.5=87.5}}}

so, equation of this line is:

{{{y= 42.5x + 87.5}}}

so,  {{{the}}}{{{ rate}}} of increase  is {{{42.5}}}