Using u=x−t: ∫0xt2sin(x−t)dt=∫0x(x−u)2sinudu.
Expanding and integrating term by term:
=x2(1−cosx)−2x(−xcosx+sinx)+(−x2cosx+2xsinx+2cosx−2)
=x2+2cosx−2.
Setting equal to x2: 2cosx−2=0⇒cosx=1⇒x=2nπ.
In [0,100]: 2nπ≤100⇒n≤π50≈15.9.
So n=0,1,2,…,15, giving 16 elements.