The general expression for a quadratic function can be written as y = a*x2 + b*x + c where a, b and c are constants.
You can find a, b and c by using the points on the graph
2 = a*(-2)2 + b*(-2) + c or 2 = 4a -2b + c
1 = a*02 + b*0 + c or 1 = c
-2.5 = a*12 + b*1 + c or -2.5 = a + b + c
You now have 3 equations for the 3 unknowns a, b and c. Can you take it from here?