Answer
$6 \ square \ units $
Work Step by Step
According to Heron's formula, the area of a triangle with sides $a,b$ and $c$ is given by:
$K=\sqrt{s(s-a)(s-b)(s-c)}$; where $s=\dfrac{a+b+c}{2}$
Therefore, $s=\dfrac{4+5+3}{2}=6$
In order to find the area of a triangle we will have to substitute the data into Heron's formula.
$K=\sqrt{6(6-4)(6-5)(6-3)}=6 \ square \ units $