First of all, you should have a model: for example: Y = x1*exp(-a1)+x2^a2+x3*a3;

Second of all, there are bunch of matlab functions that you can use:
1). Minimize the squared errors of (Y-x1*exp(-a1)-x2^2-x3) -- fmincon or fminsearch if you have constraint.
2). Minimize the same statistics aforementioned by lsqnonlin or lsqnonneg, depending on the coefficients that you want to estimate.

Hopefully this will help.

By the way, there are many examples on the website of mathworks.com so that you would walk through to get a better idea of what kind of question that you are estimating and what methods you need to apply.

Good luck.

工具箱里有吧

谢谢楼上二位！