The maximum amplitude for an amplitude modulated wave is found to...

{A}_\max = {A}_{c} + {A}_{m} = 12

{A}_\min = {A}_{c} - {A}_{m} = 3

$⇒ {A}_{c} = \dfrac{15}{2}$ and ${A}_{m} = \dfrac{9}{2}$

Modulation index $= \dfrac{{A}_{m}}{{A}_{c}} = \dfrac{\dfrac{9}{2}}{\dfrac{15}{2}} = 0 . 6$

$⇒ x = 1$