a) \(-\left(-253\right)+178-216+\left(-156\right)-\left(-21\right)\)
\(=253+178-216-156+21\)
\(=431-216-156+21\)
\(=215-156+21\)
\(=59+21\)
\(=80\)
b) \(-\left(-239\right)+115+\left(-27\right)+\left(-125\right)-121\)
\(=239+115-27-125-121\)
\(=354-27-125-121\)
\(=327-125-121\)
\(=202-121\)
\(=81\)