Consider triangle ABC:
We know that AB=BC=7 cm, AC=5cm.
Let's draw height BD. As long as AC=BC , the height BD is also a median: AD=DC=AC/2=2.5
Consider triangle BDC, its right angled. Let's find BD using Pethagorean theorem:
Hence BD=sqrt(BC^2-DC^2)=sqrt(7^2-2.5^2)= approx. 6.54
Now we can find area: