Ola Maria

x^4 - 13x^2 + 36 = 0

(x^2 - 4)*(x^2 - 9) = 0

x^2 = 4

x = -2

x = 2

x^2 - 9 = 0

x^2 = 9

x = -3

x = 3

pronto

(-3,3,-2√1. 2√2

X^4-13x^2+36=0 ........=> resolva como do 2º grau

D=b²-4.a.c= (-13)²-4.1.36=169-144=25.....=> VD=5

x=-b+VD/2a=13+5/2=18/2=9.....=> x'=V9 = -3 e 3

x=-b-VD/2a= 13-5/2=8/2= 4.......=> x''=V4 = -2 e 2

===> S: {-3, 3, -2, 2}