$a)5^4\cdot 5^6\cdot 5^2=5^{4+6+2}=5^{12}$

${\left(-2\right)}^7\cdot {\left(-2\right)}^5\cdot {\left(-2\right)}^3={\left(-2\right)}^{7+5+3}={\left(-2\right)}^{15}$