Answer
$36[cos166^{\circ}+i\sin166^{\circ}]$
Work Step by Step
$if\\ $
$z_1=r_1(cosθ_1+isinθ_1)\\$
$and\\$
$z_2=r_2(cosθ_2+isinθ_2)\\$
$then\\$
$z_1z_2=r_1r_2[cos(θ_1+θ_2)+isin(θ_1+θ_2)]$
$given\ problem\\$
$9(cos 115° + i sin 115°) .4(cos 51° + i sin 51°))\\$
$here\ r1=9,r2=4,θ1=115^{\circ},θ2=51^{\circ}\\$
$=9∗4[cos(115^{\circ}+51^{\circ})+isin(115^{\circ}+51^{\circ})\\$
$=36[cos166^{\circ}+i\sin166^{\circ}]$
