Let L1 and L2 be two lines in the plane, with equations y = m1x + c1 and y = m2x + c2 respectively. Suppose that they intersect at an acute angle θ. Show that

$\displaystyle \tan\theta = |\frac{m_1-m_2}{1+m_1m_2}| $

Help please, I've drawn a diagram but I don't really know how to proceed.

Thanks in advance