That expression can not be factored.  The number '49' depends on the prime number '7' (7*7=49) and there is no way to factor the expression in a way that ends up with '-14d-49'.  You could always use the 'SImplify' tab and enter in "what is the most likely factored expression" and see that type of expression. 

(d-7)^2

(d-7i)^2 where 'i' is the imaginary number and the only way to end up with something similar to '-14d-49'