Recall the following property of exponents:
[tex]a^b\cdot a^c=a^{b+c}[/tex]
[tex]\implies 216^{-3k}\cdot 216^{-2k}=216^{-3k-2k}=216^{-5k}[/tex]
Now, notice that 6² = 36 and 6³ = 216, which means that
[tex]216^{-5k}=36^{2k-1} \iff (6^3)^{-5k}=(6^2)^{2k-1}[/tex]
Recall another property of (real) exponents:
[tex](a^b)^c=a^{bc}[/tex]
[tex]\implies(6^3)^{-5k}=6^{3(-5k)}=6^{-15k}\text{ and }(6^2)^{2k-1}=6^{2(2k-1)}=6^{4k-2}[/tex]
So we have
[tex]216^{-5k}=36^{2k-1} \iff 6^{-15k}=6^{4k-2}[/tex]
Since both sides are equal powers of 6, that must mean that the exponents must be equal, so
[tex]-15k=4k-2[/tex]
Solve for k :
[tex]-15k-4k=(4k-4k)-2[/tex]
[tex]-19k=-2[/tex]
[tex]k=\dfrac{-2}{-19}=\boxed{\dfrac2{19}}[/tex]
