\[\boxed{\text{5.\ 48}}\]
\[\textbf{а)}\ ( - c + b) - (b - c) =\]
\[= - c + b - b + c = 0\]
\[\textbf{б)}\ (a - b) - ( - b - a) =\]
\[= a - b + b + a = 2a\]
\[\textbf{в)}\ ( - 8 - n) - (7,9 - n) =\]
\[= - 8 - n - 7,9 + n = - 15,9\]
\[\ \]