begin{multline*}
Time = a + b * h1(x) + c * h2(x) + d * h3(x) + e * h4(x) \
 where   htextsubscript{1,2,3,4}(x) =
    Bigg{
    begin{tabular}{cc}
    x  & x > 0 \
    0  & x $leq$ 0
    end{tabular}
end{multline*}

documentclass{article}
usepackage{amsmath}
begin{document}
begin{multline*}
mathrm{Time} = a + b times h_1(x) + c times h_2(x) + d times h_3(x) + e times h_4(x) \
 text{where}  h_{1,2,3,4}(x) =
    begin{cases}
    x  & x > 0 \
    0  & x leq 0
    end{cases}
end{multline*}
end{document}

documentclass{article}
usepackage{amsmath}
begin{document}
We have
[mathrm{Time} = a + b times h_1(x) + c times h_2(x) + d times h_3(x) + e times h_4(x)]
where
[h_{1,2,3,4}(x) =
    begin{cases}
    x  & x > 0 \
    0  & x leq 0
    end{cases}]
end{document}

documentclass{article}
usepackage{amsmath}
begin{document}
We have
[mathrm{Time} = a + b times h_1(x) + c times h_2(x) + d times h_3(x) + e times h_4(x)]
where
[h_{1,2,3,4}(x) =
    begin{cases}
    x  & text{if $x > 0$} \
    0  & text{otherwise}
    end{cases}]
end{document}

documentclass[12pt]{article}
usepackage{amsmath}
begin{document}

begin{flalign}
  mathrm{Time} = & , a + b ast h1(x) + c ast h2(x) + d ast h3(x) + e ast h4(x) &nonumber\
  mathrm{where} & , h_{1,2,3,4}(x) =
    begin{cases}
    x  & x > 0 \
    0  & x leq 0
    end{cases}
end{flalign}

end{document}