] for A to be singular matrix ad=bc Case 1: exactly 1 number is used ⇒‌4C1 ways Case 2: exactly 2 numbers is used ⇒‌4C2 ways Case 3: exactly 3 numbers used ⇒ none will be singular. Case 4: exactly 4 numbers is used ⇒ab=cd⇒2×9=3×6 ⇒‌4C1×2!=8 matrix. ∴ Total ways ⇒4+6×4+8=36 matrices.