Question 5799
You surely meant {{{80x-16x^2}}}, right?

For height at 4 seconds, use "4" instead of "x":

*[invoke explain_simplification "80*4-16*4^2"]

so, 64 is the height at 4 seconds.

To factor, I use my quadratic solver:

*[invoke quadratic "x", -16, 80, 0]

factoring is -16x*(x-5). 

If you again use 4 instead of x:

*[invoke explain_simplification "-16*4*(4-5)"]

so, the answer is again 64!