Algebra Questions, Need help!

Need help with a few questions please,

1. x^3+3x^2=81x+243

2. 729-1331x^3

3. x^4-10x^3+21x^2=0

Thank you very much if you can answer! :)

x^3 + 3x^2 - 81x - 243 = 0
x^2(x + 3) - 81(x + 3) = 0
(x^2 - 81)(x + 3) = 0
(x + 9)(x - 9)(x + 3) = 0
x = {-9, -3, 9}


729 - 1331x^3
-(1331x^3 - 729)
-(11x - 9)(121x^2 + 99x + 81)

x^4 - 10x^3 + 21x^2 = 0
x^2(x^2 - 10x + 21) = 0
x^2(x - 3)(x - 7) = 0
x = {0, 3, 7}