It is very simple just use the Heron's formula :---
As we know the three sides of the eqillateral triangle are 10 cm each, as all the three sides of an equillateral triangle are equal.
First we have to find the semi-perimeter of the triangle which is Sum of all sides / 2
And in this case the semi-perimeter is 15 cm.
So when we know the semi-perimeter, what we have to do is just put the value of the semi-perimeter instead of s in the heron's formula.
And in the place of a, b and c we have to put 10 cm as the value as all sides are equal.
After putting all values the answer would be 649.51 cm square (approx)