Answer
$3.6$ mi/h
Work Step by Step
Let us consider that $v_1=$ Velocity of the swimmer and $v_2=$ Velocity of the river.
Let $R$ be the resultant velocity of the swimmer.
Thus, we have
$R=v_1+v_2$
Here, $v_1=2i; v_2=-3j$
Then $R=2i-3j$
and
$|R|=\sqrt{(2)^2+(-3)^2}=\sqrt {4+9}=\sqrt {13}$
or, $|R|=3.6$ mi/h
