f(x)=ax^2+bx+c
f(2x)+f(3x+1)
=4ax^2+2bx+c+a(3x+1)^2+b(3x+1)+c
=13ax^2+(6a+5b)x+2c+a^2+b=13x2+6x-1
a=1
6a+5b=6
2c+a^2+b=-1
b=0,c=-1
f(x)=x^2-1
f(x)=ax^2+bx+c
f(2x)+f(3x+1)
=4ax^2+2bx+c+a(3x+1)^2+b(3x+1)+c
=13ax^2+(6a+5b)x+2c+a^2+b=13x2+6x-1
a=1
6a+5b=6
2c+a^2+b=-1
b=0,c=-1
f(x)=x^2-1