If you look at the quadratic formula:
x=(-b±√(b^2-4ac))/(2a)
We can say that -b=5, b=-5
If I read it right you had ±2√7 which is equal to ±√28, and we know b=-5 so:
±√(25-4ac)=±√28
And since 3=2a, a=1.5 making the above:
±√(25-6c)=±√28
25-6c=28
-6c=3
c=-0.5
So a=1.5, b=-5, and c=-0.5 thus
y=1.5x^2-5x-0.5
We can check this by reapplying the quadratic formula to the equation above
x=(5±√(25-4(1.5)(-0.5))/3
x=(5±√(25+3))/3
x=(5±√28)/3
x=(5±√(4*7))/3
x=(5±2√7)/3
