Equation and Cubicroot/cube root in form

Hey again

i had some trouble makeing my program finding squareroot on a userdefined number, but thanks to somebody i got it fixed! thx! :D now, iom in need of help again.. what about the cubicroot/cube root how do i find that

and can i make my program able to solve user defined equations so the user just types en aquation in "textbox1" and the program solves the equation

//Martin J.