TeX source:

\dfrac{dq_s}{dt} =\left\{ \begin{array}{lll} p_s−p_m, & {0 ≤ t ≤ t_s} \\ −p_m, & {t_s ≤ t ≤ T_s} \\ 0, & {T_s ≤ t ≤ T} \end{array} \right.