Question 929132
The quick way through is to know the slope-intercept form for a linear equation.  Look at the graph showing x=0, y=b, the y-intercept.  You don't know it, but at least you are given the slope and one data point.


{{{y=mx+b}}}
{{{y=-0.3x+b}}}
{{{y+0.3x=b}}}
{{{b=0.3x+y}}}
substitute the known given point.
{{{b=0.3*15+24.5}}}
{{{b=4.5+24.5}}}
{{{b=29}}}
The equation for your line is 
{{{highlight(y=-0.3x+29)}}}


Just find y for x=8.
{{{y=-0.3*8+29}}}
{{{y=-2.4+29}}}
{{{highlight(y=26.6)}}}