I have a very long equation and this causes me some problems. I want it to look the following way:

left side = right side\\

+continuation of right side\\

=right side again modified\\

+continuation of the modified right side\\

=result

I first used the align environment:

\begin{align*} \pi^{M}(s^{M},f^{M},x^{M}) :&= 12s^{M}+14f^{M}-(s^{M})^2-(x^{M}-4)^2-(f^{M})^2-x^{M}f^{M}-(x^{M})^2 &=12\times6+14\times\dfrac{48}{7}-6^2-\left(\dfrac{2}{7}-4\right)^2-\left(\dfrac{48}{7}\right)^2 -\dfrac{2}{7}\times\dfrac{48}{7}-\left(\dfrac{2}{7}\right)^2\\ &=69,14 \end{align*}

However, the problem is, that I'm doing this in a presentation and the column width is limited. Despite of that {align} doesn't adjust the expression to the margins of the slide (by braking the expression before a + or a - sign and continuing it on the next line), but makes it go out of the whole page area.

I thought of multline, however, the problem is, that multline aligns the first row left the second to last but one rows centered and last row right. This works well, if one has an expression of the type a+b=c+d+e+f+g+h=j, where there's only one row between initial row and result. However, in my case, I have some mathematical transformation (meaning an additional row) and so I need the equal sign of the row: "12\times6+14\times\dfrac{48}{7}-6^2-\left(\dfrac{2}{7}-4\right)^2-\left(\dfrac{48}{7}\right)^2

-\dfrac{2}{7}\times\dfrac{48}{7}-\left(\dfrac{2}{7}\right)^2" to be aligned under the first equal sign. So now I use this version:

\begin{align*} \pi^{M}(s^{M},f^{M},x^{M}) :=& 12s^{M}+14f^{M}-(s^{M})^2-(x^{M}-4)^2-(f^{M})^2\\ &-x^{M}f^{M}-(x^{M})^2\pause\\ =&12\times6+14\times\dfrac{48}{7}-6^2-\left(\dfrac{2}{7}-4\right)^2-\left(\dfrac{48}{7}\right)^2\\ &-\dfrac{2}{7}\times\dfrac{48}{7}-\left(\dfrac{2}{7}\right)^2\pause\\ =&69,14\pause \end{align*}

Does anybody have better ideas? Thank you