Your answer is 4.7
Using the Pythagorean Theorem, you can find the length of the missing side:
[tex] a^{2}+b^{2}=c^{2}\\(3)^{2}+b^{2}=(5.6)^{2}\\9+b^{2}=31.36\\b^{2}=22.36\\b=\pm \sqrt{22.36}[/tex]
Length cannot be negative, so your answer is:
[tex] b=\sqrt{22.36}=4.728636...\\\\b=4.7[/tex]