Question 824129
This same question has occurred a few weeks ago.


Identify two data points (x, y).  Since the description indicates the function to be height of the candle, use x as burn time, and y as height.  The two points then are (7, 22.8) and (21, 14.4).  


We want y=mx+b equation form.
{{{m=(14.4-22.8)/(21-7)=-8.4/14=highlight(-3/5)}}}

Rearranging to have b, 
b=y-mx
Use EITHER identified point
{{{b=14.4-(-3/5)21=14.4+(3/5)21=highlight(27)}}}
'
The equation for height versus burn time is {{{highlight(y=-(3/5)x+27)}}}.


Your objective to finish is find y when x=12.