Answer
$$4\sqrt{13}.$$
Work Step by Step
We have $r(t) =2ti-3tk$, and so $r'(t) =2i-3k$.
We calculate the length by integration:
$$length=\int_{11}^{15}\sqrt{4+9}dt=\sqrt{13}t|_{11}^{15}=4\sqrt{13}.$$
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