assuming you want quadratic
when x=0, y=-16
y=ax^2+bx+c
-16=0+c
-16=c
when x=2, y=0
0=a(2)^2+2b-16
0=4a+2b-16
16=4a+2b
8=2a+b
when x=4, y=0
0=a(4)^2+4b-16
0=16a+4b-16
16=16a+4b
4=4a+b
we has 2 equations
8=2a+b
4=4a+b
multiply first by -1 and add to 2nd
-8=-2a-b
4=4a+b +
-4=2a+0b
-4=2a
-2=a
sub back
4=4a+b
4=4(-2)+b
4=-8+b
12=b
y=ax^2+bx+c
y=-2x²+12x-16
