Answer
$y=-2.35x+255$
Work Step by Step
From the graph the two points are approximately $(0,255),(100,20)$, thus if $y=mx+a$, then $m=\frac{y_2-y_1}{x_2-x_1}=\frac{20-255}{100-0}=\frac{-235}{100}=-2.35$.
So $y=-2.35x+a$, hence plugging in $(x,y)=(0,255)$: $255=-2.35\cdot0+a$, thus $a=255$. So the equation of the line is $y=-2.35x+255$
