[tex]\\ \bull\tt\dashrightarrow P=P_1+P_2[/tex]
[tex]\\ \bull\tt\dashrightarrow P=+3D-2D=+1D[/tex]
Now
[tex]\\ \bull\tt\dashrightarrow f=\dfrac{1}{P}[/tex]
[tex]\\ \bull\tt\dashrightarrow f=\dfrac{1}{1}[/tex]
[tex]\\ \bull\tt\dashrightarrow f=1m[/tex]
