[
text{Sous contraintes}left{
    begin{array}{ll}
    eq1 & conditions1\
    eq2 & conditions2\
    eq3 & conditions3\
    end{array}
    right.
]

begin{empheq}[left = empheqlbrace]{align}
    eq1 && conditions1\
    eq2 && conditions2\
    eq3 && conditions3
end{empheq}