can you help me with the algebra in this. how did they solve and get 250 shares for y

Margin Trading - Margin Cait - You need to bring the account to at least MM (some brokers may require IM). Sell some shares Balance Sheet after sale of y shares Stock $50,000-50*y Borrowed Equity $35,000- 50"y $15,000 15000 Margin= 50000 - 50 y = 40% y = - 250 shares