[lexi.vm]

100 mL is exactly Neutralized by 150 mL of 0.1M Ca(OH)2

answer is 0.1 M H3PO4

How is this supposed to be done?

[Babcock_Hall]

Does the question provide the identity of the acid, which is phosphoric acid in this case?  Assuming that the answer to my question is yes, I would treat this as a standard problem in stoichiometry.  Please show us your work (which is a forum rule), and we may be able to help.