Answer
$bx^2+x(3a+c)+(2a-b+c)$
Work Step by Step
We are given:
$T(1)=x+1 \\
T(x)=x^2-1\\
T(x^2)=3x+2$
We obtain:
$$T(ax^2+bx+c)=T(ax^2)+T(bx)+T(c)\\
=aT(x^2)+bT(x)+cT(1)\\
=a(3x+2)+b(x^2-1)+c(x+1)\\
=3ax+2a+bx^2-b+cx+c\\
=bx^2+x(3a+c)+(2a-b+c)$$