Quoting a market in the spread A minus B

You make markets in two products, written bid @ ask: product $A$ is quoted $4 \text{ @ } 5$ and product $B$ is quoted $10 \text{ @ } 12$. You are now asked to quote the combination $A - B$ (long one $A$, short one $B$), consistent with your existing markets.

If your market in $A-B$ is $X \text{ @ } Y$, report $Y^2 - X^2$.

$-39$. To buy $A-B$ you would buy $A$ at your bid and sell $B$ at your ask, so the bid is $4-12=-8$; to sell it you sell $A$ at your ask and buy $B$ at your bid, so the ask is $5-10=-5$. The market is $-8 \text{ @ } -5$ and $25-64=-39$. Simply subtracting bid from bid would give a crossed market.