​ need help fast please.

First  blank  we have   1 /tan u   

And 1/ tan u  = 1 / (sin u / cos u)  =  cos u / sin u

So now we have 

cos u  * sec u  *  cos u  / sin u  =       cos u  * (1 /cos u)  *  (cos u /sin u) 

(The red part goes into the  second blank)

And this   evaluates to     cot u   .....which is what we are trying to show  !!!!