Here are the formulas for what you are asking for.
P = s1 + s2 + s3
Area = height * base / 2
Givens
s1 = 12.6
s2 = 6.3
s3 = 7.4
height = 4.5
base = 7.4
The area is just slightly different than it normally is. The reason is the height. It is not directly connected to the base. The construction is called the base extended. See the givens for what that is.
Solution
<em>Perimeter</em>
P = s1 + s2 + s3 Substitute givens into the formula