Area of a triangle is 1/2 Base x Height.
This is an isosceles triangle with base sqrt(28), so you can split it in half to form 2 right angled triangles with base = sqrt(28) / 2 , hypotenuse = sqrt(70) and height = x.
Use pythagorus to get the height of the right angled triangle:
x = sqrt[70 - (28/4)] = sqrt(63)
Therefore area of Isosceles triangle = 2 x area of right angled triangle = 2[1/2 * (sqrt28)/2 * sqrt(63)]
= 21 units ^2
Gahh CPhill beat me to it...