You have to multiply and add!
We know that the equation of the line is in the form 
. To find b, we can use the fact that (6,-5) lies on the line, meaning that both sides of the equation are equal when x=6 and y=-5.
Hence, the equation of the line is 
, and its graph looks something like follows.
After 45 minutes you will have painted 4*45=180 square feet.
so the total number of painted area, y, is:
y=180+x*(3+4) after the 45 minutes
where x is the number of minutes AFTER THE 45 minutes:
if y is 1000 then
1000=180+x*7
820=x*7
x=117.14
so 117 minutes, or one hour and 57 minutes.
