f(1) = 1^2-a = 1-a=-1,所以a=2;  ff(0) = f(-2) = 2*(-2)+3 = -1;
x<0时,f(x)<3,所以f(x)=4的解取在x>=0上.x^2-a=4在x>=0上有解,即x^2-a的最小值<=4,即：-a<=4,所以a>=-4