Find the volume of a right prism with height h=10 cm, if: the base is a trapezoid with height htr = 3 cm and bases of 2 cm and 5cm

The volume of the prism is:
 V = Ab * h
 Where,
 Ab: base area
 h: prism height
 The area of the base is:
 Ab = (1/2) * (B + b) * (htr)
 Where,
 B: major base
 b: minor base
 htr: height of the trapeze
 Substituting values:
 Ab = (1/2) * (5 + 2) * (3)
 Ab = 10.5
 Then, the volume is:
 V = (10.5) * (10)
 V = 105 cm ^ 3
 
 The volume of the right prism is:
 V = 105 cm ^ 3

What the heck don’t understand