Answer: C
Step-by-step explanation:
By the geometric mean theorem,
[tex]\frac{y}{5}=\frac{4}{y}\\ \\ y^{2}=20\\\\y=\sqrt{20}[/tex]
So, by the Pythagorean theorem,
[tex]4^{2}+(\sqrt{20})^{2}=x^2\\\\36=x^2\\\\x=\boxed{6}[/tex]
