(1/2)*b*h, where b is the base and h is the height. Further, for an ISOSCELES triangle, height can be found by the following formula:
h = sqrt[(b^2 - (1/2)*a^2)]
The area of an isosceles triangle can also be found using trigonometry, but I am assuming you are not quite up to that level yet.
Another formula you can consider using is Heron's formula.
Given a triangle with sides of length a, b, c, the area of the triangle is:
A = sqrt(s*(s-a)*(s-b)*(s-c)),
where s = (a + b + c)/2.