Firstly, write a balance chemical equation of the neutralization reaction

H\(_3\)PO\(_4\)  +  3NaOH → Na\(_3\)PO\(_4\)  +  3H\(_2\)O

Given:

V\(_A\) = ? C\(_A\) = 0.1M N\(_A\) = 1 

V\(_B\) = 45cm\(^3\)  C\(_B\) =  0.2M  N\(_B\) = 3

Using the formula,

\(\frac{{C_A}{V_A}}{{C_B}{V_B}}\) = \(\frac{N_A}{N_B}\)

V\(_A\) = \(\frac{{C_B}{V_B}{N_A}}{{C_A}{N_B}}\)

V\(_A\) = \(\frac{{0.2}\times{45}\times{1}}{{0.1}\times{3}}\)

V\(_A\) = 30cm\(^3\)