Part 1.
For a vector
[tex]\vec{v}=\langle x,y\rangle[/tex]
its magnitude is written as
[tex]|\vec{v}|[/tex]
and is given by
[tex]|\vec{v}|=\sqrt[]{x^2+y^2}[/tex]
This formula comes from Pythagorean theorem:
Therefore, the answer for part A is:
[tex]|\vec{v}|=\sqrt[]{x^2+y^2}[/tex]
Part B.
Now, we can use the above result into the given vector. Then, the magnitude of vector u is
[tex]\begin{gathered} |\vec{u}|=\sqrt[]{4^2+7^2} \\ |\vec{u}|=\sqrt[]{16^{}+49} \\ |\vec{u}|=\sqrt[]{65} \\ |\vec{u}|=8.0623 \end{gathered}[/tex]
Then, the answer for part B is:
[tex]|\vec{u}|=8.0623[/tex]