7. 3z+2t t+3s -- --= zt st

7. $$ \frac{3z+2t}{zt} - \frac{t+3s}{st} = \frac{s(3z+2t) - z(t+3s)}{stz} = \frac{3sz+2st-tz-3sz}{stz} = \frac{2st-tz}{stz} $$

Ответ: $$ \frac{2st-tz}{stz} $$