-3x²-x+1
=-3(x²-x/3)+1
=-3(x²-x/3+1/36-1/36)+1
=-3(x²-x/3+1/36)+3/36+1
=-3(x-1/6)²+13/12
-3(x-1/6)²≤0
-3x²-x+1≤13/12