You can use
cos(theta) = u · v / ║u║║v║
that will come out to:
u = 2 x (-4) = -8
v = 3 x (-8) = - 24
u · v = -16
║u║ = [tex]\sqrt{(2)^{2} + (-4)^{2} }[/tex] = [tex]\sqrt{20} = 2\sqrt{5}[/tex]
║v║ = [tex]\sqrt{(3)^{2} + (-8)^{2} } = \sqrt{73}[/tex]
cos(theta) = -16/2 [tex]\sqrt{365}[/tex]
and to find (theta), you can now do
theta = (cos^-1) ( -16/2 [tex]\sqrt{365}[/tex])
I don't have a calculator, but you can solve that final problem to find theta
