in a triangular prism, B usually stand for the triangular base of the prism. It is the area of the triangle.
Area of a right triangle = ab/2
a = long leg ; b = short leg
Given measures are:
a = 4 yd ; b = 3 yd
A = (4 yd * 3 yd)/2 = 12 yd² / 2 = 6 yd² is the value of B.
the hypotenuse is 5 yd but it is not needed to get the area of the right triangle base. 7 yd is the measure of the height of the triangular prism.