Question 814097
The description gives two data points, (x,y) where x is time and y is tallness of the candle.  These two points are (2, 8) and (4, 5.5).  These two points define a line in variables x and y.


That desired line:
All using slope-intercept form, {{{m=(5.5-8)/(4-2)=-2.5/2=-5/4}}};
{{{y=mx+b}}}
{{{b=y-mx}}}
Use either point from the problem description,
{{{b=8-(-5/4)*2}}}
{{{b=8+(5/4)*2}}}
{{{b=8+5/2=8+2.5}}}
{{{b=10.5}}}


The equation for the line is {{{highlight(y=(5/4)x+10&1/2)}}};
Again, y is tallness or height of the candle in inches, and x is time in hours after burning (at the wick).  The question is about how much time to burn to become 4.5 inches.  JUST SOLVE THE EQUATION FOR x, and let y=4.5.


The whole situation could have been analyzed using x for tallness and y for burn time, but normally, most people would use time as an independant variable and something else as the dependant variable.