Approximation factor for graph problems
An approximation algorithm for a maximization algorithm has an approximation ratio of $\alpha \geq 1$ if whenever the true answer is $m$, the algorithm is promised to return a solution with value at least $m/\alpha$.

In your case, $m = 13$ and $\alpha = 1.5$, and so by definition, the algorithm must return a clique whose size is at least $13/1.5$. Since the size of the clique is an integer, if a clique is of size at least $13/1.5 \approx 8.6$, then it is in fact at least $\underline{\quad}$ (fill in the blanks).