Multiple Nonlinear Regression in MATLAB

Code:

clc
clear all
close all
x1=[0.5 2 10 0.5 2 10 0.5 2 10];%mean water velocity
x2=[0.15 0.15 0.15 0.3 0.3 0.3 0.5 0.5 0.5];%depth
y=[0.48 3.9 57 0.85 5 77 0.8 9 92];%mass transfer coefficient...
...of the dissolved oxygen
x1=log10(x1);
x2=log10(x2);
y=log10(y);
n=length(x1);
a=[ones(n,1) x1' x2'];
c=pinv(a)*y';