3A + 2B + C = 5,2a + b-3c = 1,3a + b-7c = m, a, B and C are expressed by algebraic expressions containing m respectively

3A + 2B + C = 5,2a + b-3c = 1,3a + b-7c = m, a, B and C are expressed by algebraic expressions containing m respectively

A = 7c-3, B = 7-11c are obtained from the first two formulas, and C = [M + 2] / 3 is obtained by substituting the third formula
therefore
a=〔7m+5〕/3
b=-〔11m+1〕/3
c=〔m+2〕/3