Is it homework ? Here are some hints:
- if you remove the first element to all elements of an affine space, you get a vector space
- this vector space has a basis
- turn this basis into a matrix M (by columns)
- the lines of the matrix A you are looking for are a basis of the right kernel of M (write the things down on a paper to get convinced)
- you get the b by plugging any element of your affine space in the equation