Equation of the form ax^4+bx^2+c=0, where a!=0 is called biquadratic. Biquadratic equations is solved with method of introducing new variable: setting x^2=y, we obtain quadratic equation ay^2+by+c=0.
Example. Solve equation x^4+4x^2-21=0.
Let x^2=y, then we obtain quadratic equation y^2+4y-21=0. Solutions of this equation are x=-7 and x=3.
Recall, that x^2=y, therefore we have set of equations: x^2=-7,x^2=3.
First equation doesn't have roots, second has two roots: sqrt(3) and -sqrt(3). These two roots will be roots of the initial equation.