\documentclass[12pt,a4paper]{article} \usepackage[fleqn]{amsmath} \begin{document} \begin{alignat*}{2} \text{Then } x &= a_0y \text{ for some } y \in D\\ &= a_0uy \begin{alignedataligned}[t]{1} & = a_0yu\\ &=& = xu \end{alignedataligned} \end{alignat*} \end{document}
\documentclass[12pt,a4paper]{article} \usepackage[fleqn]{amsmath} \begin{document} \begin{alignat*}{2} \text{Then } x &= a_0y \text{ for some } y \in D\\ &= a_0uy \begin{alignedat}[t]{1} & = a_0yu\\ &= xu \end{alignedat} \end{alignat*} \end{document}
\documentclass[12pt,a4paper]{article} \usepackage[fleqn]{amsmath} \begin{document} \begin{alignat*}{2} \text{Then } x &= a_0y \text{ for some } y \in D\\ &= a_0uy \begin{aligned}[t] & = a_0yu\\ & = xu \end{aligned} \end{alignat*} \end{document}