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.