0.400 g of a solid base X(OH)2 was dissolved in 50.0 mL of water. The solution was neutralized with 65.80 mL of 0.150 mol/L HCl. Calculate the molar mass of the base.

Do I need to set up a dilution equation? I'm not sure what to do.

X(OH)2 + 2HCl =-> XCl2 + 2H2O

mols HCl = M x L = ?
Using the coefficients in the balanced equation, convert mols HCl to mols X(OH)2

Then mols X(OH)2 = grams/molar mass. You know mols and grams, solve for molar mass.