C6H12 = 6x12 + 6x1 = 78. The equation indicates that 2x78 = 156g benzene will produce 6542kJ. Using proportions you can then calculate that x/6542kJ = 6.4g / 156g x = 268.39 kJ = 268390 J
heat = mass x ΔT x 4.18J/g° ΔT = 268390 J / (5691g x 4.18J/g°) = 11.28 °C