Multivariable piecewise function fit matlab

Hello there,

I would like to fit experimental data with an asymmetric Gaussian function: that is, a Gaussian like peak with different widths on the left and right hand sides.

I am mostly interested in the mean location of the peak, or b:

gaussEqn = 'a*exp(-((x-b)/c)^2)+d';

but also on the 'c' of both sides to do an average between them.