Adolphin is 42 feet underwater and ascends at a constant rate for 14 seconds until it reaches the surface

At 0 seconds, the dolphin is 42 feet below the surface: 
x = 0, y = -42 
The y intercept = -42 (0,-42) 

x = the elapsed time in seconds 
y = depth of dolphin relative to the surface 

The x intercept occurs when y = 0, and the y intercept occurs when x = 0 

After 14 seconds, the dolphin is even with the surface (0 below the surface): 
x = 14, y = 0 

The x intercept = 14 (14,0) 

The slope = (y2 - y1)/(x2 - x1) = 42/14 = 3. 

Therefore, the formula for this line, in slope intercept form is y = 3x - 42.