Answer
$4\ cis\pi$
Work Step by Step
$if\\ $
$z_1=r_1(cosθ_1+isinθ_1) = r_1cis(\theta_1)\\$
$and\\$
$z_2=r_2(cosθ_2+isinθ_2)= r_2cis(\theta_2)\\ $
$then\\$
$z_1z_2=r_1r_2\ cis(θ_1+θ_2)]\\$
$given\ problem\\$
$2\ cis(\frac{3\pi}{4}).2\ cis(\frac{\pi}{4})\\$
$here\ r_1=2,r_2=2,θ_1=(\frac{3\pi}{4}),θ_2=(\frac{3\pi}{4})\\$
$=2*2 cis[(\frac{3\pi}{4})+(\frac{\pi}{4})]\\$
$=4\ cis(\frac{3\pi+\pi}{4})$
$=4\ cis(\frac{4\pi}{4})$
$=4\ cis\pi$
