a) \(S = \left( {45 - 3756} \right) + 3756\)
\( = 45 - 3756 + 3756\)\( = 45 + \left( { - 3756} \right) + 3756 = 45 + 0 = 45\)
b) \(S = \left( { - 2021} \right) - \left( {199 - 2021} \right)\)
\( = \left( { - 2021} \right) - 199 + 2021\)\( = \left[ {\left( { - 2021} \right) + 2021} \right] - 199 = 0 - 199\)\( = - 199\)