I can't figure out how to do this question. simplify the imaginary number in the picture above please

Answer:
-i(6/7)√2
Step-by-step explanation:
[tex]\displaystyle -\sqrt{-\frac{72}{49}}=-\sqrt{-1}\sqrt{\frac{36}{49}}\sqrt{2}\\\\=-i\frac{6}{7}\sqrt{2}[/tex]