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Calculati :
a) 2x -5y+ 8x+5y =
b) 7a + 8c - b+ 14c-6a-21c =
c) 5m-6+3n +9m-4-5n-7m+2n-2m=
d) )6x+18y-14z) -(12x+15y+27z) - ( 5x -10y-40z) =