Can anyone show me how to solve this?

D = (0, 5 2/3)

You are given two points and the slope of the line. You can write the point-slope form of the equation for it, then solve for the y-intercept.

For slope = 2/3 and point (-4, 3), the equation of the hypotenuse line can be written as ...

... y = (2/3)(x -(-4)) +3

... y = (2/3)x +8/3 + 3

... y = (2/3)x +17/3

So, the y-intercept (point D) is at y = 17/3 = 5 2/3. Of course, the x-coordinate of D is 0, so the ordered pair is ...

... D = (0, 5 2/3)