writing equation of absolute value in latex format

As starting point:

documentclass{article}
usepackage{mathtools}

begin{document}
[
I_{mathrm{avg}} =
    begin{dcases}
    1   & text{ if } 0 < delta < T_c, text{ without ICI},    
    frac{2T_u+3T_c}{3(T_u+T_c)} + sum_{l=1}^{K-1}frac{T_u}{(pi l)^2(T_u+T_c)}
        & condition
    end{dcases}
]
end{document}

• the writing of the second condition I left to you
• solution consider how such math structures are usually written
• math environment dcases is defined in the mathtools package, which is enhanced of the amsmath package defined by American Mathematics Association, so it is considered as standard way of writing advanced mathematics
• amsmath define cases, which in comparison to dcases left math inside cases structure in inline mode (as is written in the text)
• for basic how to write math in LaTeX you can learn from some introductory text about LaTeX and writing mathematics, for example in the The Not So Short Introduction to LaTeX and in the wiki Mathematics.