Theory:

A general quadrilateral can be split into two triangles by drawing one of its diagonals.
qudrilateral main.PNG
 
Area of quadrilateral: The area is half the product of the diagonal and sum of heights of two triangles in it.
qudrilateral.PNG
 
Area of quadrilateral = 12dh1+h2.
  
where \(d\) denotes the length of diagonal.
  
where h1 and h2 is denoted as heights.
 
Proof: Area of quadrilateral = 12dh1+h2.
 
Area of quadrilateral \(ABCD =\) (Area of \(△ ABC\)) \(+\) (Area of  \(△ ACD\)).
 
\(=\) 12AC×h1 \(+\) 12AC×h2.
 
\(=\) 12ACh1+h2.
 
\(=\) 12dh1+h2.
 
Therefore, Area of quadrilateral \(=\) 12dh1+h2 \(square units\).