If the leading coefficient of a quadratic function is negative then the graph of the function extends from?

I wouldn't use the phrase "extends from."  If the leading coeff. is neg, then the graph opens downward.  Without more info we do not know the max of this fn.  If we did know it, we could state that the graph max is (value) and that the graph "extends downward from this value."

The linear equation is: y=ax+b