Concept:
If two adjacent sides of a rectangle are represented by vectors, the area can be calculated using the product of magnitudes when the sides are perpendicular.
Step 1: Given:
\[
\vec A=3\hat i+4\hat j
\]
and
\[
\vec B=\hat i+3\hat j
\]
Step 2: Magnitude of \(\vec A\):
\[
|\vec A|=\sqrt{3^2+4^2}
\]
\[
|\vec A|=\sqrt{9+16}=5
\]
Step 3: Magnitude of \(\vec B\):
\[
|\vec B|=\sqrt{1^2+3^2}
\]
\[
|\vec B|=\sqrt{1+9}=\sqrt{10}
\]
Step 4: Area of rectangle:
\[
\text{Area}=|\vec A|\cdot|\vec B|
\]
\[
=5\sqrt{10}
\]
\[
\boxed{5\sqrt{10}\text{ units}}
\]