Find de C value for
I would appreciate any suggestions on how to solve this problem, thanks.
The problem is symmetric. There is as much probability above $C$ as there is below $-C$
The total amount of probability available is 1 and thus
$P[Z < -C ] = P[Z > C] = \dfrac{1-0.95}{2} = 0.025$
Now just look up in your table or your software the inverse CDF of the normal distribution at $0.025$
and this will be the value of $-C$
I getSpoiler: