Solve the equation after making an appropriate substitution x4 - 6x +8=0 Determine the appropriate substitution using the new variable u: u = Write the new quadratic equation using the variable u: (Type an equation. Type your answer in standard form.) Solve the new quadratic equation for the variable u. u = (Simplify your answer. Type an integer or a simplified fraction. Use a comma to separate answers as needed. The solution set to the original equation is (Simplify your answer. Type an exact answer, using radicals as needed. Use a comma to separate answers as needed.)