Since each process takes kx time, for any process to be completed, it needs to go through k cycles of scheduling.
Now, since we were asked about 1st process, we don't need to calculate for all k cycles, we only need to calculate for (k-1) cycles and plus x for last cycle.
i.e (k-1) cycles + x
We have n process in each cycle and each process takes x time, therefore time taken for (k-1) cycles is
n(k-1)x
total execution time for 1st process is
=> n(k-1)x + x
=> x(n(k-1) + 1)
=> x(nk - n + 1)
which is the answer