by defintion

here you have

so you can just say it is the derivative of cos(a) and just state that as the limit, namely, -sin(a)

if you would rather calculate it manually, note that an alternate definition for the derivative is as follows

it is easier to find this particular limit in that form

getting your limit in that form is not as hard as you think

let , then becomes

now proceed using the addition formula for cosine to simplify

(note, you will need your special trig limits here)