1. Calculate the mass and moles of H2O driven off:
6.85 g - 3.13 g = 3.72 g H2O / 18.0 g/mol = 0.207 moles H2O
2. Calculate moles Sr(OH)2 remaining:
3.13 g Sr(OH)2 / 121.6 g/mol = 0.0257 moles Sr(OH)2
3. Calculate the ratio of moles H2O / moles Sr(OH)2:
0.207 / 0.0257 = 8.0
So, your experimental determination of x is 8.0