The triangle is a right triangle.
The unknown side is the hypotenuse. The length of the hypotenuse is calculated to be:
[tex]c=\sqrt[]{a^2+b^2}[/tex]where a and b are the other two sides.
Therefore, the unknown side is calculated to be:
[tex]\begin{gathered} a=6.4 \\ b=3.5 \\ c=\sqrt[]{6.4^2+3.5^2} \\ c=\sqrt[]{40.96+12.25} \\ c=\sqrt[]{53.21} \\ c\approx7.3 \end{gathered}[/tex]The missing side is about 7.3 cm long.