a)
\(\left(-8537\right)+\left(1975+8537\right)\)
\(=\left(-8537\right)+1975+8537\)
\(=1975\)
b)
\(\left(35-17\right)+\left(17+20-35\right)\)
\(=35-17+17+20-35\)
\(=20\)
c)
\(273+\left[-34+27+\left(-273\right)\right]\)
\(=273-34+27-273\)
\(=27-34\)
\(=-7\)