Angle Between Lines Drawn Through Rectangle

Given a rectangle with two straight lines drawn through, we can find the angle between the lines. If the gradients of the lines are
$m_1$
and
$m_2$
with
$m_1 \gt m_2$
then the angle between the lines is
$tan^{-1} (m_1) - tan^{-1}(m_2)$
.

The line
$l_1$
goes along 7+3=10 and up 6-3=3. The gradient is
$\frac{3}{10}$
.
The line
$l_2$
goes along 3-6=-3 and up 3+5=8. The gradient is
$\frac{8}{-3}$
.
The angle
$\alpha$
is
$tan^{-1}( \frac{8}{-})- tan^{-1}(\frac{3}{10})=93.9^o$
to 1 decimal place.