Código
double a,b,c,raiz,x1,res1,res2;
a=Double.parseDouble(this.getTextField4().getString());
b=Double.parseDouble(this.getTextField5().getString());
c=Double.parseDouble(this.getTextField6().getString());
raiz=Math.sqrt(x1);
x1=(b*b)+4*a*c;
res1=((-1*b)+raiz)/2*a;
res2=((-1*b)-raiz)/2*a;
this.stringItem2.setText(String.valueOf(res1));
this.stringItem3.setText(String.valueOf(res2));