We first compute the no. of moles of each element in the compound. So first, the Si is said to have 33% by mass in the solution. So we solve: (0.33 g Si/1 g sol'n)(1 mol Si/28 g Si)(170 g sol'n/1 mol sol'n) = 2.00 moles of Si. Then next we solve for the moles of F. We follow the same procedure: (0.67 g F/1 g sol'n)(1 mol F/19 g F)(170 g sol'n/1 mol sol'n) = 5.99 or 6.00 moles of F. So we have Si2F6.