Left and centre alignment when using subequation and align environments

I have {amsmath},{amssymb} packages. The code :

\begin{subequations}
\begin{align}
u_1(x) = \frac{-2 (m + n  + k)}{y (x-b)^2 + z} + \frac{a+b+c+d+e+f+g+h+j+k+l+m+n+ .... (let us make it so long) }{x+y+z} .
\end{align}
\end{subequations}

Well, the point is one of the subequations is so long comparing to the other.

What I need are;

(1) Centre all subequautions (without using \qquad or \quad).

(2) Align left all sub equations (without using \qquad or \quad).

Also, I need to tag each equation such as (a,b) (automatically not manually tagging).

Tried to use {gathered}, {alignat} and {aligned} environments with different context. But, it seems to me they do not provide all the conditions I have. Any proper way will be highly appreciated!

With environment align you need to add ampersands, which determine, where equations are aligned (usually at equal sign). Below are two cases of taging of your equation. See, if one is what you after: \begin{subequations}
\begin{align}
u_0(x) & = \frac{x} {y (x-b)^2 + z} ,               \label{eq:1a} \\
u_1(x) & = \frac{-2 (m + n  + k)}{y (x-b)^2 + z} .  \label{eq:1bd}
\end{align}
\end{subequations}
or
\begin{align}
u_0(x) & = \frac{x} {y (x-b)^2 + z} ,               \label{eq:a}\tag{a} \\
u_1(x) & = \frac{-2 (m + n  + k)}{y (x-b)^2 + z} .  \label{eq:b}\tag{b}
\end{align}
See \eqref{eq:1a} and \eqref{eq:1b} or \eqref{eq:a} and \eqref{eq:b} \dots\\[1em]
\textbf{Addendum}: with environment  'gather` equations are centered:
\begin{gather}
u_0(x) = \frac{x} {y (x-b)^2 + z} ,               \label{eq:c}\tag{c} \\
u_1(x) = \frac{-2 (m + n  + k)}{y (x-b)^2 + z} .  \label{eq:d}\tag{d}
\end{gather}
• thanks for your answer. However, it is not what I am looking for. I will edit my question .One of the equations should be longer than the other. Then, I am wondering what could be used to show both equations at the center. – Meva Jan 30 '17 at 19:48
• well, than you looking for gather? – Zarko Jan 30 '17 at 19:49