The polynomial as per the given of the problem is f(x)=3x²-21x+30
Find the roots of this polynomial:
x₁=[-b+√b²-4ac)]/2a and x₂ =[-b-√b²-4ac)]/2a
Plug in : x₁ = [21+√(21²-4(3)(30)]/6 = [21+√81}/6 ; x₁ = 5
and x₂ = [21-√(21²-4(3)(30)]/6 = [21-√81]/6 ; x₂ = 2
Remark: I don't understand where did you bring √5 , it should be 5