By titrations it is found that 27.5 milliliters of 0.155 M NaOH are required to neutralize 25.0 milliliters of HCl. Calculate the concentration of the hydrochloric acid solution.

I think you may use the equation M1V1= M2V2 but I am not sure how to calculate it.

Thanks for the help.

you are right
use M1V1= M2V2
M1= 0.155 M NaOH
V1=27.5 milliliters
M2=???
V2=25.0 milliliters of HCl
M2 = M1V1/V2