Pouvez vous m'aider svp merci

bonjour

a = (4/5 : 2/3) x (1/2-3/4)
a = (4/5 x3/2) x (2/4-3/4)
a = 12/10 x -1/4
a = 6/5 x -1/4
a = -6/20
a = - 3/10