Csdrhrt

documentclass{article}



usepackage{amsmath}



begin{document}



begin{align*}

y ={}& frac{1}{n!} leftlbrace int + f(a) right. \

     &left. vphantom{int} + [h(u)+phi(x)] + C rightrbrace 

end{align*}

end{document}

How to align the plus sign with fraction? Thanks.

Alignment is not really necessary and multline might do the job. If you feel that alignment is important, here are three proposals.

documentclass{article}



usepackage{amsmath}



begin{document}



The following aligns the + with the fraction

begin{equation*}

begin{split}

y ={} & frac{1}{n!} biggllbrace int + f(a) \

      & {} + [h(u)+phi(x)] + C biggrrbrace

end{split}

end{equation*}

but with a slight offset; with the following the

offset is removed

begin{equation*}

begin{split}

y ={} & frac{1}{n!} biggllbrace int + f(a) \

      & {mspace{-medmuskip}} + [h(u)+phi(x)] + C biggrrbrace

end{split}

end{equation*}

However, I'd align with the integral sign 

begin{equation*}

begin{split}

y = frac{1}{n!} biggllbrace &!int + f(a) \

      & {mspace{-medmuskip}} + [h(u)+phi(x)] + C biggrrbrace

end{split}

end{equation*}



end{document}

try

begin{align*}

y &= frac{1}{n!} leftlbrace int + f(a) right. \

  &phantom{=} left. vphantom{int} + [h(u)+phi(x)] + C rightrbrace 

end{align*}    

You do not require alignment here (which is why you are needing phantom etc to hide the alignment point) you just have a line that needs to be broken (so I assume you have a narrow text width) something like:

documentclass[twocolumn,a5paper]{article}



usepackage{amsmath}



begin{document}



begin{multline*}

y =frac{1}{n!} Bigllbrace int + f(a)  \

    {} + [h(u)+phi(x)] + C Bigrrbrace 

end{multline*}

end{document}