Fitting problem with data in workspace vectors

I am very new to MATLAB. I am currently working on a problem that requires me to fit unknowns in an equation to a graph plot. Since the unknown is an input to a bessel's function and its zero crossings are also part of the equation, I am unable to form an equation for the unknown. The equation is of the form y= a*exp(-b*x)+(1-a)*c wherein I know the values of both x and y and c is another equation in which I need to solve for the unknowns at every value of (x, y). All the data is stored in workspace vectors, which I am unable to import to cftool or optimization tool. Please help