The California Almond Growers have at most $3600$ boxes of almonds to be shipped from their plant in Sacramento to Des Moines and San Antonio. The Des Moines market needs at least $600$ boxes, while the San Antonio market must have at least $1000$ boxes. Let $x=$ the number of boxes to be shipped to Des Moines and $y=$ the number of boxes to be shipped to San Antonio. Complete parts $($a$)$ and $(b).$

$($a$)$ Write a system of inequalities to express the conditions of the problem.