15.Efectuați calculele: a) (-18x²) : (-3x) + (−21x³): (+3x²) - (-56x4): (-7x³); b) (-64a³b²) : (−16a²b) + (+72a³b²) : (-6a²b) - (+63aºbº) : (−21a³b³); c) (-48x^y4): (+6x²y²) + (-24x³y): (+6x³y2²)-(-45x6y³): d) (−72a¹bª) : (−8a²b²) + (+36a³b³) : (−6ab) + (−54a³b4) : (−18ab²). (+5x¹y³);​