3x+6x+9x .................... to n terms = 3/2n(n+1)x

3x + 6x + 9x + ......n terms 
they are in AP
therefore, the common difference is 3x

sum = n/2 [2.3x + (n - 1) 3x]
        =3*n (2 + n - 1)/2
       =3/2 nx (1 + n)

Hence proved !