if the line L1, passes through the first pair of points, L2 through the second pair, find the angle from L1 to L2. given are;...

You need to remember the equation that helps you to find
the angle between the given lines and , such
that:

represents the slope of the line

represents the slope of the line

Since the line l_1 passes through the points and
yields:

Since the line passes through the points and
yields:

2/(-4) => m_2 = -1/2

You may substitute - for
and - for in equation of tangent , such
that:

= (-5/2)/(5/2) = -1

You need to remember that the tangent
of an angle is negative if the angle is obtuse, hence or
.

Hence,
evaluating the angle between the lines l_1 and l_2 yields
.