a)

-3 × -5 = -3 - (-5 × -3) - -5

-3 × -5 = 15

-3 - (-5 × -3) - -5 = -3 - 15 - -5 = -13

15 ≠ -13 ∴ the equation is false

b) Just do the same prosses but swap the numbers

-5 × -3 = -5 - (-3 × -5) - -3

-5 × -3 = 15

-5 - (-3 × -5) - -3 = -5 - 15 - -3 = -17

15 ≠ -17 ∴ the equation is false

c) x*y=y*x is true, but I don't get how doing x*y=x-(y*x)-y is supposed to prove if x*y=y*x is true or false since they're different equations.