Ok any more details or is that the question?
The vol. of a cube of side length s is V=s^3.
So, if the side length is 5a+4b, then the volume of the cube is
V(a, b) = (5a+4b)^3. This could, of course, be expanded, using the binomial theorem, but there's no point in doing so.
I drew it out below (don't you just love my effort? XD)
You're gonna use the sine trig function to find the length of the shadow
sin26 / 1 = 43 / x; now you cross multiply
sin26 x = 43; divide both sides by sin26
x = 43 / sin26
x = 98 which is the shadow
Hope this helps!
The banner would be 10feet 3inches