18. \( \left(5 x^{5}\right)\left(3 y^{6}\right)\left(3 x^{2}\right) \)

To simplify \( \left(5 x^{5}\right)\left(3 y^{6}\right)\left(3 x^{2}\right) \), start by multiplying the coefficients (the numbers in front). You have \( 5 \times 3 \times 3 = 45 \). Next, combine the like terms for \( x \). You have \( x^5 \) and \( x^2 \), so you add the exponents: \( 5 + 2 = 7 \). The \( y^{6} \) remains as is since there are no other \( y \) terms. Thus, the simplified expression is: \[ 45 x^{7} y^{6} \]