If A = [1 2 2; 2 1 2; 2 2 1], then what is A² - 4A equal to? (where I₃ is the identity matrix of order 3)

A² row 1: [1·1+2·2+2·2, 1·2+2·1+2·2, 1·2+2·2+2·1] = [9, 8, 8]. Row 2: [2+2+4, 4+1+4, 4+2+2] = [8,9,8]. Row 3: [2+4+2, 4+2+2, 4+4+1] = [8,8,9]. So A² = [9 8 8; 8 9 8; 8 8 9]. A² - 4A = [9-4, 8-8, 8-8; 8-8, 9-4, 8-8; 8-8, 8-8, 9-4] = [5 0 0; 0 5 0; 0 0 5] = 5I₃.