To solve this expression, we need to apply the power rule of exponents.

(x^3)^6 = x^(3*6) = x^18

Therefore, the expression simplifies to:

x^18 * x^4

To simplify this further, we need to add the exponents when multiplying like bases:

x^(18+4) = x^22

Therefore, the final simplified expression is x^22.