finding equation of a parabola

hiii i just need some help finding the equation of a parabola that has the points (-a,0), (a,0) and (0,a/2)

this is based on a volume of revolution question,

a hemispherical bowl of radius a units is filled with water to a depth of a/2 units. find by integration the volume of the water

i just really need some help with this because i tried simultaenous equations for the equation of the parabola and i got

(-1/4)a(x^2)+(1/4)x+(a/2)

can anyone tell me if this equation is right for the parabola because i dont think it is

thankyou to anyone who can help me !! =)