function of area of isosceles triangle

The equal sides of an isosceles triangle are 2. If x is the base, express the area as a function of x.

The area of a triangle is 1/2 BH and the height of an Isosceles triangle I learned is :

H=square root of b^2-1/4a^2 (b is base, a is either of the equal sides)

So I got as an answer :

A = 1/2 X (x^2-1/4(2)^2)

Im pretty sure this is wrong. I keep trying to check it but Im getting it wrong. The official answer is:

A = 1/4X SQRT of 16-x^2

Dont see how to get that answer, can someone please help?