Answer :
The norm of a point in the cartesian coordinate (x, y) is given by
[tex]\mleft\Vert(x,y\mright)\mleft\Vert=\sqrt[]{x^2+y^2}\mright?[/tex]For points c(x, y), the points will be (cx, cy), hence
[tex]\mleft\Vert(cx,cy\mright)\mleft\Vert=\sqrt[]{(cx)^2+(cy)^2}\mright?[/tex]From the question, we have
u = (5, -12)
c = -3
Therefore,
[tex]\begin{gathered} cu=-3(5,-12) \\ =(-3\times5,-3\times-12) \\ cu=(-15,36) \end{gathered}[/tex]||cu|| is given by
[tex]\begin{gathered} \mleft\Vert cu\mleft\Vert=\sqrt[]{(-15)^2+36^2^{}}\mright?\mright? \\ =\sqrt[]{225+1296} \\ =\sqrt[]{1521} \\ \mleft\Vert cu\mleft\Vert=\mright?\mright?39 \end{gathered}[/tex]Therefore, ||cu|| equals 39.
OPTION B is correct.