Emily = x
Samantha = x + 2
Lauren = x + 4
x + (x + 2) + (x + 4) = 42
x = 12
Thus:
Emily = 12
Samantha = 14
Lauren = 16
Since we have to solve this system graphically, we'll have to draw both lines.
To draw a line, simply build a little tables with two x,y values: for the first line we have

So, you can draw the points (0,3) and (1,2), connect them, and you'll have the graph of the line 
For the second line we have

So, you can draw the points (0,5) and (1,6), connect them, and you'll have the graph of the line 
Finally, we have to check where the two lines cross. If your drawing is accurate enough, you'll see that the point you're looking for is (-1, 4)
Yes,
Every 2 minutes = 3 inches of water, so that means every minute = 1.5 inches of water.
So in 10 minutes:
10*(1.5) = 15 inches of water.
So in 10 minutes the tub will fill an additional 15 inches of water which is exactly how much you need to fill up the remaining tub (3+15 = 18)
It would be 50.24 ft
BTW in this case I used 3.14 for pi