For this question, we need formulate a new equation in this case D^2 (which is the distance squared). From there on to get the minimum value, we just need to find the stationary value through d/dx. You can go and differentiate again to find d2/dx2 and show it is the minimum value but I didn’t want my working to extend to 2nd page :P. With the stationary value, you can substitute back to D^2 and get the distance. Hope this helps ;)