Fit data curves by other data curves

I have a 1D array of data (named 'Density'). I know it is a linear combination of two other 1D data arrays I have (named 'wavefun1' 'wavefun2').

That is, for every point in 'Density'

    % Density(i) = a1*wavefun1(i)+a2*wavefun2(i)

How can I find a1,a2 that fit best? is there a tool for that because I didn't find.