HCl+NaOH---> NaCl+H2O
the 2 reactives are monovalent . the NaOH is 3 times as concentrated as HCl
So you need 3 volumes HCl 2M to neutralize 1 volume NaOH 6M
and for 75 ml NaOH 6M you need 75*3= 225mL HCl 2M
second question HBr+NH4OH--> NH4Br+H2O
as before 1 HBr for 1NH4OH is needed IF they have the same molarity
Here you need 40/10 =4 more HBr than NaOH
The molarity of NaOH is 4*0.5 =2M
