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2 Answers

Assuming that the argument of the cosine is pi/5 rather than n/5 (the pi character (π) from the special characters set above (Ω) comes out looking like an n).  I'll use the word pi rather than the symbol:
  1. First compute the cos(pi/5) = cos(0.628 radians) = 0.809
  2. Next multiply cos(pi/5) by 4 = 4*(.809) = 3.236
  3. Now subtract 3.236 from 4: 4-3.236 = 0.764
 
The priorities of the operations are:
  1. Do the math inside the argument of the cos first, e.g. π/5
  2. Then take the cosine of the argument
  3. Then multiply the value of the cosine by the constant 4
  4. Then subtract all that from 4
4 - 4cos(n/5) = 0
 
I assume you are trying to find the value for n
 
subtract 4 from each side
 
-4cos(n/5) = -4
 
divide by 4
 
cos(n/5) = 1
 
in the interval 0, 2PI
 
cos (n/5) = 1 when (n/5) = 0   or n = 0
 
and cos(n/5) = 1 when n/5=2PI  or n = 2pi/5
 
if there is no restriction on the interval
 
Then cos(n/5) =1 when n/5= 2*x*PI and      n = 2/5*x*PI   where x are all the interger values