The distributive property helps perform this multiplication.
... (x-5)(x² -2x -6)
... = x(x² -2x -6) -5(x² -2x -6) . . . . . second factor times each term in the first factor
... = x³ -2x² -6x -5x² +10x +30 . . . use the distributive property again
... = x³ -7x² +4x +30 . . . . . . . . . . . combine like terms