The formula of the Pythagorean Theorem is
[tex]\begin{gathered} a^2+b^2=c^2 \\ \text{ Where c is the hypotenuse and} \\ a,b\text{ are the sides of the triangle} \end{gathered}[/tex]Graphically,
So, in this case, you have
[tex]\begin{gathered} a=3 \\ b=7 \\ c=\text{?} \\ a^2+b^2=c^2 \\ \text{ Replacing} \\ (3)^2+(7)^2=c^2 \\ 9+49=c^2 \\ 58=c^2 \\ \text{ Apply square root to both sides of the equation} \\ \sqrt[]{58}=\sqrt[]{c^2} \\ 7.62=c \end{gathered}[/tex]Therefore, the missing side measurement is 7.62 units.