Chapter 2 - The Logic of Compound Statements - Exercise Set 2.1 - Page 38: 39

$(num-orders\geq50)\ OR\ num-instock\leq300)\ AND\ (50\gt num-orders\geq75\ OR\ num-instock\leq500)$

To negate an AND statement, change it to an OR statement with both components negated. To negate an OR statement, change it to an AND statement with both components negated.