p80 q11 ex18c

question:

A and D are two towns on the same side of the main road BC. A road APD is to be made to join A and D to the main road. where should the location of P be in order that the length of the road is its smallest? you can only use differentiation method.

my working:

let be the length of the road, x be the distance of BP

i cannot find the value of x when dl/dx = 0 . thanks!