documentclass[twoside,14pt]{extarticle}
usepackage{array}
newcolumntype{L}[1]{>{raggedrightarraybackslash}m{#1}}

usepackage{amsmath}

begin{document}

    renewcommand{arraystretch}{2.4}
    setlengthtabcolsep{1.5tabcolsep}
    setlengtharrayrulewidth{1.5pt}
    begin{center}
        begin{tabular}{!{vrule width 1.5pt} L{75mm} !{vrule width 1.5pt} L{75mm} !{vrule width 1.5pt}}
            hline
            As $displaystyle fleft(xright) rightarrow +infty$, & $displaystyle dfrac{1}{fleft(xright)} rightarrow$
            \ hline
        end{tabular}
    end{center}

end{document}

documentclass[twoside,14pt]{extarticle}

usepackage{array}
usepackage{cellspace}
setlength{cellspacetoplimit}{8pt}
setlength{cellspacebottomlimit}{8pt}
newcolumntype{L}[1]{>{raggedrightarraybackslash}S{p{#1}}}

usepackage{amsmath}

begin{document}

    setlengthtabcolsep{1.5tabcolsep}
    setlengtharrayrulewidth{1.5pt}
    begin{center}
        begin{tabular}{!{vrule width 1.5pt} L{75mm} !{vrule width 1.5pt} L{75mm} !{vrule width 1.5pt}}
            hline
            As $displaystyle fleft(xright) rightarrow +infty$, & then $displaystyle dfrac{1}{fleft(xright)} rightarrow$
            \ hline
        end{tabular}
    end{center}

end{document}