Exercise 4.3.24

The derivatives of $\parallel Ax-b{\text{∥}}^2=x^T{A}^{T}Ax-2b^{T}Ax+b^{T}b$ (this last term is constant) are zero when $2A^{T}Ax=2A^{T}b$, or $x={\left(A^{T}A\right)}^{-1}{A}^{T}b$.