Solving an equation in simulink

Hi, I have an equation that looks like x^3+b*x^2+c*x+d=0, where d is the output of a PI controller which varies continuously and b and c are constants, I need to find the value of x with the variation in c in SIMULINK, I tried roots but even if I made sure that there will not be any complex roots, matlab still showed complex signal mismatch error. Please help me to get through this problem....