5x+40 factors to 5(x+8). Notice how distributing the 5 back through to each term in the parenthesis gives
5 times x = 5x
5 times 8 = 40
So 5*(x+8) = 5*x+5*8 = 5x+40
Therefore, the factors are 5 and (x+8).
The dimensions of the sandbox are 5 feet by (x+8) feet.
We don't know the numeric value of (x+8) since we don't know the value of x, so we leave it as is.