DifferentiatING both sides two times w.r.t.

   ……….(ii)

Again differentiating w.r.t. ,  ……….(iii)

Multiplying eq. (i) by 3 and subtracting eq. (ii) from it, we get

 ……….(iv)

Again multiplying eq. (ii) by 3 and subtracting it from eq. (iii), we get

 ……….(v)

Now, eq. (v) + 2 eq. (iv) gives,

 , which is required differential equation.