Can someone help me with this problem? please explain.

The units attached to the numbers can help you out. They work just like variables when you go to add, subtract, multiply, or divide.

You have weight (5.76 pounds) and density (0.08 pounds/in³). The units of volume (in³) are in the denominator of the density, but we want them in the numerator of a number that gives the volume of the brick. At the same time, we want to cancel units of pounds. We can do all that by dividing weight by density.

  volume = weight/density
  volume = (5.76 pounds)/(0.08 pounds/in³) = (5.76/0.08) in³ = 72 in³

We know the volume of a rectangular prism (brick) is given by the product of its length, width, and height. We now have enough information to find the height.
  volume = length*width*height
  72 in³ = (8 in)*(2.25 in)*height . . . . . substitute known numbers
  72 in³/((8*2.25) in²) = height . . . . . .. solve for height
  72/18 in = height = 4 in . . . . . . . . . . . do the arithmetic. (in³)/(in²) = in

The height of the brick is 4 inches.