Area of common shapes
A rectangle is length times width, a triangle is half of that, and a circle is pi times the radius squared.
A worked example
A circle of radius 6 has area 3.14 x 6² = 3.14 x 36 = 113.0 and circumference 2 x 3.14 x 6 = 37.7.
The mistake people actually make
In πr², square the radius before multiplying by pi, and use the radius rather than the diameter. Circumference is 2πr, with no square.