Q: Solve by matrix method: 3x + 2y = 12, x - y = 1. A = [[3,2],[1,-1]], b = [12,1]. det = -3 - 2 = -5. A⁻¹ = (-1/5)[[-1,-2],[-1,3]] = [[1/5,2/5],[1/5,-3/5]]. x = A⁻¹b = [(12+2)/5, (12-3)/5] = [14/5, 9/5]. Check: 3(2.8)+2(1.8) = 12 ✓.