Answer: 4
Step-by-step explanation
The parabola is of the form y = ax^2 since the vertex passes through the origin.
Plug in x = 28 and y = 49, and solve for 'a', to get a = 1/16
Therefore, the equation of the parabola is y = (1/16)x^2
What we can do is multiply both sides by 16 to get 16y = x^2
Compare this to the form 4py = x^2
We see that 16y = 4py leads to p = 4.
p is the focal distance, which is how far it is from the vertex to the focus.
I used GeoGebra to confirm the answer.