Draw a quarter circle of radius 10√2, chose a point P on the circle, draw a line from the centre C to P and then complete the triangle as in my diagram below.
Now chose a different point P and construct a triangle the same way.
All three of these triangles meet your requirements and yet they have quite different areas. In fact the area of the last triangle is almost zero. If you only have the measurement of one side you can construct many different triangles with quite different areas.