= [ x3 ( 1 + x + x2 + x3 + ....................... ) ]3
= x9 ( 1 + x + x2 + x3 +.......................)3
= x9 (1 / ( 1 - x ) )3
= x9 ( 1 / ( 1 - x )3 )
= x9 ( ( 1 - x )-3 )
= x9 ( 1 + (3)x + ( (3*4) / (1*2) ) x2 + ( (3*4*5) / (1*2*3) ) x3 +...................................)
= x9 + 3x10 + 6x11 + 10x12 +........................................................
Answer = 10