For x< 5, \frac{x^2- 25}{x- 5}= \frac{(x- 5)(x+ 5)}{x- 5}= x+ 5 so the limit, as x goes to 5 from below is 5+ 5= 10.
The limit, as x goes to 5 from below, of f is 10d+ c.
Cosine is always between -1 and 1 and x- 5 goes to 0 as x goes to 0 so the limit, as x goes to 5 from above, of this function...