Actually Welcome to the Concept of the Trigonometry.
here, we use the Linear pair property of the adjacent angles.
We know that, all the adjacent angles in a linear pair add to get 180°
so we get as,
=> (n+6) +90°+(2n+3) =180°
=> (n+6) +(2n+3) =180-90
=> 3n+9 = 90
=> 3n= 90-9
=> 3n = 81
hence, n = 81/3
=> n = 27°
thus the value of n is 27° .