Answer:0.75 ms$^{-2}$

According to Newton's second law, $F = ma$

Given, $a1 = 3 m/s^2$ and $a2 = 1 m/s^2, \rightarrow F = m1 \times 3 = m2 \times 1 \rightarrow \large\frac{m1}{m2} $$= \large\frac{1}{3}$

$\Rightarrow m2 = 3 m1$

Now, if the masses are combined into one entity, $F = (m1+m2) a \rightarrow F = (m1 + 3m1) a = 4m1 \times a$

This is equal to $F = m1a1 \rightarrow m1a1 = 4m1a \rightarrow a = \large\frac{m1 \times 3}{4 \times m1} $$= 0.75 ms^{-2}$