$a)\ \left(-\dfrac{3}{8}x^3y^2z\right) \cdot \left(-6xy^3z^5\right)$

$= \left[\left(-\dfrac{3}{8}\right) \cdot (-6)\right] \cdot \left(x^3 \cdot x\right) \cdot \left(y^2 \cdot y^3\right) \cdot \left(z \cdot z^5\right)$

$= \dfrac{18}{8}x^{3+1}y^{2+3}z^{1+5}$

$= \dfrac{9}{4}x^4y^5z^6$

$b)$ Đơn thức $\dfrac{9}{4}x^4y^5z^6$ có:

Hệ số: $\dfrac{9}{4}$

Phần biến: $x^4y^5z^6$

Bậc: $4 + 5 + 6 = 15$