Write an equation in standard form of an ellipse that is 28 units high and 20 units wide

The standard form of ellipse with vertical major axis and center at the origin is:
x^2/b^2+y^2/a^2=1, a>b
2a=length of the major axis=28
a=14
a^2=196

2b=length of minor axis=20
b=10
b^2=100

x^2/196+y^2/100=1

making y the subject we get

y=5/7(196-x²)^0.5