NOTE: If your equation is a Quartic (in the form of ax4 + bx3 + cx2 + dx + e = 0), simply enter in the coefficients like normal. However, If your equation is a Cubic (in the form of: ax3 + bx2 + cx + d = 0), enter in a 0 into the x4 field. Then enter the coefficients as normal. If the equation is a Quadratic (in the form of ax2 + bx + c = 0) enter in a 0 into both the x4 AND x3 fields. Then enter the coefficients as normal.
Transpose the terms so the equation is set equal to 0 (ax4 + bx3 + cx2 + dx + e = 0 [where a, b, c, d, and e are the real, known, numerical coefficents]). Plug in the coefficents into each box. If the equation has a 'missing term' (for example, no x2 term), then enter it as a zero. After you have entered in the equation, press 'Solve' and your 4 solutions will show under 'x1' , 'x2' , 'x3' and 'x4'.
Click on this text for an example (15x4 - 58x3 + 8x2 - 80x + 64 = 0)