a)

$8^7=8\cdot 8\cdot 8\cdot 8\cdot 8\cdot 8\cdot 8$

b)

${\left(-\frac{3}{4}\right)}^3=\left(-\frac{3}{4}\right)\cdot \left(-\frac{3}{4}\right)\cdot \left(-\frac{3}{4}\right)$