Volume of rectangular prism = length * width * height
Volume of triangular prism =( length * height * base )/2
Given,
Volume of rectangular prism = 18 + volume of triangular prism

18x = 18+14x
4x=18
x= 4.5 ft
So volume of rectangular prism = 6* 4.5* 3 = 81 ft³
Volume of triangular prism = (7* 4.5 * 4) / 2 = 63 ft³