求救!! 1.f( x + h )....f( x )有咩

求救!!
1.f( x + h )....f( x )有咩關係?
點解f( x ) = 3x^2 + 5x -6 代入去[f(x+h)-f(x)]/h

結果係[3(x+h)^2+5(x+h)-6]-[3x^2+5x-6]

2. a SIR 講過 當f(x)=4
f(1)=4 + 0*1 =4
f(x+h)=4+0*(x+h)=4
唔明點解會咁..

3.一條數唔識做~ lim [1/x-1/4]/2(x-4)
x>4