In the given diagram reading of sping balance will be

Dear student,
Here reading in the spring balance will be equal to the tension produced in the string.
$$\begin{gathered} For 3 kg mass \\ T-mg=ma \\ T- 3 g=3a \\ T-30 N =3a..1 \\ For mass of 6 kg \\ Mg-T=ma \\ 6g-T=6a \\ 60 N-T=6a...2 \\ Adding 1 and 2 \\ 30 = 9a \\ a = \frac{10}{3} m/s^2 \\ Now substituting value of a in equation 1 \\ T-30 N =3\times \frac{10}{3}N \\ T= 40 N \\ So reading of spring balalance is 40 N. \\ Correct answer is 2. \end{gathered}$$