Question 200825
b^4 +13b^2 +36 =0
let  a = b^2
a^2 +13a +36 =0
(a+9)(a+4) =0
subst  back
(b^2 +9)(b^2 +4) =0
.
b^2 +9 =0,,,,,   b^2 = -9,,,b=  +/-  3i
.
b^2 +4 = 0,,,,,,b^2 = -4,,,,,,,,,b= +/-  2i