a)([tex] 2x^{2}+4xy) + ( y^{2} - 2xy) - ( x^{2} + 4 y^{2} - 5xy) + ( x^{2} - y^{2})=[/tex]
b)[tex]11m+(3n-5m+6)-(4m-2n-8)=[/tex]
c)[tex](12 m^{2}-5m+ n^{2})-(2 n^{2}+6m-3 m^{2})-(12 m^{2} - mn)=[/tex]