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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:

First compute the cos(pi/5) = cos(0.628 radians) = 0.809

Next multiply cos(pi/5) by 4 = 4*(.809) = 3.236

Now subtract 3.236 from 4: 4-3.236 = 0.764

The priorities of the operations are:

Do the math inside the argument of the cos first, e.g. π/5

Then take the cosine of the argument

Then multiply the value of the cosine by the constant 4