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.
Let's say for example you wanted to graph the linear equation 
At minimum, we'll need 2 points to form this line.
To get any point of the form (x,y), we plug in a number for x to find a corresponding number for y.
For example, replace x with 0 to find that...

Therefore, x = 0 leads to y = 5. So the point (0,5) is on the line.
Then we repeat this process for other x values. Let's try x = 1

Telling us that (1,7) is also on this line. We have enough to graph this equation. Plot the two points (0,5) and (1,7). Then draw a straight line through them. Extend this line as far as you can to the left and right. The graph is shown below.
We can keep going to get other points like (2,9) and (3,11) and (4,13) and so on. A table is a way to keep track of the xy values.

Each time x goes up by 1, y goes up by 2. This directly ties to the slope of 2/1 = 2.
Answer: 
Step-by-step explanation:
By definition, the perimeter of a rectangle is:

Where "l" is the lenght and "w" is the width.
If you solve for "l":

In this case, you know that the following expression represents the perimeter of the rectangle:

And the width of that rectanle is represented wih this expression:

Therefore, based on the explained above, you can conclude that the lenght of that rectangle is given by:

Finally, simplifying the expression, you get:


we needed to eliminate w, do so by diving both sides by w and obtain the above result :)
Hope this helps.
Since U is being subtracted by 8.1, we can assume that if we added 8.1 to both sides, it should equal U
U - 8.1 = 6.35 To make it equal to U, we have to add 8.1 to both sides
The answer is,
U = 14.45