Concept:
For a second-degree equation
\[
Ax^2+Bxy+Cy^2+Dx+Ey+F=0,
\]
the angle of rotation required to eliminate the \(xy\)-term is given by
\[
\tan 2\theta=\frac{B}{A-C}.
\]
Step 1: Identify the coefficients of the quadratic terms.
Given
\[
3x^2+2xy+3y^2-18x-22y+50=0
\]
Comparing with
\[
Ax^2+Bxy+Cy^2+Dx+Ey+F=0,
\]
we obtain
\[
A=3,\qquad B=2,\qquad C=3.
\]
Step 2: Apply the rotation formula.
Using
\[
\tan 2\theta
=
\frac{B}{A-C}
\]
gives
\[
\tan 2\theta
=
\frac{2}{3-3}
=
\infty
\]
Hence
\[
2\theta=\frac{\pi}{2}
\]
Step 3: Find \(\theta\).
\[
\theta=\frac{\pi}{4}
\]
Therefore,
\[
\boxed{\theta=\frac{\pi}{4}}
\]