For Q3, the "trick" is to do the other series by visualizing the
last term as the first. Then the arithmetic progression is just
-d.
So we get (k/2) [2a + 2d(n-1)] + (k-1)d] ---- *
The other one is just the normal (k/2) [2a + (k-1)d]. ----
**
Taking * minus **, you should solve your problem directly.