Remember that
An equilateral triangle has three equal sides and three equal interior angles.
The measure of each interior angle is 60 degrees
Applying the law of sines
The area of the triangle is equal to
[tex]A=\frac{1}{2}b^2\cdot\sin (A)[/tex]where
P=3b
P=30 cm ------> given
so
30=3b
b=10 cm
angle A=60 degrees
substitute
[tex]\begin{gathered} A=\frac{1}{2}\cdot10^2\cdot\sin (60) \\ A=50\cdot\frac{\sqrt[\square]{3}}{2} \\ A=25\sqrt[\square]{3}\text{ cm2} \end{gathered}[/tex]