Use the general formula of a parabola
x2=4ay or y2=4ax
Since it states that the y axis is a tangent then it must mean that the parabola is sideways hence
y2=4ax
Now you are told that x+y+2=0 is a tangent, sub that into your general equation of a parabola then since it only touches the parabola at one point the discriminant equals to 0 then solve for a, and voila