f(x +3)=-2x-6
【解法1
令x+3=t
-2x-6=-2(x+3)=-2t
f(t)=-2t
用x替换t
f(x)=-2x
【解法2
x+3 看成整体
f(x+3)=-2(x+3)
用x替换(x+3)
f(x)=-2x