2C6H6(l)+15O2(g) ----> 12CO2(g) +6H2O(l) + 6542 kJ
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
final temp = 21 + 11.28° = 32.28°C