y= (x-3)(x-4)=x²-4x-3x+12=x²-7x+12
幂函数y=x^a(a为常数),有(x^a)'=ax^(a-1)
∴(x²)'=2x
(-7x)'=-7
常数的导数等于0
∴y'=(x²-7x+12)'=2x-7