2HBr+Mg(OH)2 -> MgBr2+2H2O

Question

how many mL of a 0.2 M HBr solution is required to react with 40 mL of a 0.40 M Mg(OH)2 solution

Answer 1

How many mols Mg(OH)2 do you have? That's mols = M x L = ?
How many mols HBr will that use. The coefficients tell you 2 mols HBr are required for every mol Mg(OH)2.

Then M HBr = mols HBr/L HBr. You know mols and M, substitute and solve for L, then convert to mL.
Post your work if you get stuck.