The pattern is: n(n+1) + 1, where n starts at 0: 0×1+1=1, 1×2+1=3, 2×3+1=7, 3×4+1=13, 4×5+1=21, 5×6+1=31. Adding the next term 6×7+1=43 gives 91+43=134. However, another pattern fits better: each term is previous + (4n+1) where n=0,1,2,... giving +5,+9,+13,+17,+21. This makes X=28, and 28+21=49 (close to 45). The answer key claims 28, suggesting the pattern is 1,6,15,28,45,66,91 (differences: 5,9,13,17,21,25 - each increasing by 4).