Concept:
In partnership, profit sharing ratio depends on:
\[
\text{Investment} \times \text{Time}
\]
Step 1: Find the investment ratio.
Given:
\[
X:Y=7:5
\]
Let:
\[
X=7k,\quad Y=5k
\]
Also:
\[
Y:Z=3:4
\]
So:
\[
5k:Z=3:4
\]
\[
Z=\frac{20k}{3}
\]
Step 2: Multiply by time.
X and Y invested for 12 months:
\[
X=7k\times12=84k
\]
\[
Y=5k\times12=60k
\]
Z joined after 3 months, so invested for 9 months:
\[
Z=\frac{20k}{3}\times9=60k
\]
Thus profit ratio:
\[
84:60:60
\]
Simplify:
\[
7:5:5
\]
Step 3: Find each share.
Total ratio:
\[
7+5+5=17
\]
Total profit:
\[
20400
\]
Value of one part:
\[
\frac{20400}{17}=1200
\]
So:
X’s share:
\[
7\times1200=8400
\]
Z’s share:
\[
5\times1200=6000
\]
Step 4: Find the difference.
\[
8400-6000=2400
\]
Thus, the required answer is:
\[
\boxed{2400}
\]