x^4-25x^2+60x-36=0

X³ (x-2) + 2x² (x-2) - 21x (x-2) + 18 (x-2) = 0

(x-2) (x³+2x²-21x+18) = 0

(x-2) [x² (x-1) + 3x (x-1) - 18 (x-1) ]=0

(x-2) (x-1) (x²+3x-18) = 0

(x-2) (x-1) [x (x-3) + 6 (x-3) ]=0

(x-2) (x-1) (x-3) (x+6) = 0

x-2=0⇒x=2

x-1=0⇒x=1

x-3=0⇒x=3

x+6=0⇒x=-6