Formula is 
Area = ½ × b × h
b = base
h = vertical height

how to solve it 
Since the area of a parallelogram is , the area of a triangle must be one-half the area of a parallelogram. Thus, the formula for the area of a triangle is:
  or  where  is the base,  is the height and · means multiply.The base and height of a triangle must be perpendicular to each other. In each of the examples below, the base is a side of the triangle. However, depending on the triangle, the height may or may not be a side of the triangle.
To find the area of a triangle, multiply the base by the height, and then divide by 2. The division by 2 comes from the fact that a parallelogram can be divided into 2 triangles. 

Formula of area of triangle=√s(s-a)(s-b)(s-c)
And a=42cm , b=34cm , c=20cm
Now s=a+b+c/2
Putting these values in area of triangle
so, area of triangle=√s(s-a)(s-b)(s-c)