Answer
$-6$
Work Step by Step
Let $u=\left\langle a,b\right\rangle$ and $v=\left\langle c,d\right\rangle$ be two vectors and $k$ be a real number, then:
$ku=\left\langle ka,kb\right\rangle$ and $u\cdot v=ac+bd$
Now for $u=\left\langle -2,1\right\rangle$ and $v=\left\langle 3,4\right\rangle$ we have
$3u=3\left\langle -2,1\right\rangle=\left\langle 3(-2),3(1)\right\rangle=\left\langle -6,3\right\rangle$ and
$(3u)\cdot v=\left\langle -6,3\right\rangle\cdot \left\langle 3,4\right\rangle=(-6)(3)+(3)(4)=-18+12=-6 $
Hence, $(3u)\cdot v=-6$ .
