number 15, 17

for number 15, i just thought of it like this, x^n+1 if n is odd will have (x+1) as a factor, it will be (x+1)(y{whatever y may be}) is divisible by (x+1).

number 17, x^3=4x+4, which equals to x^3-4x-5, so i graphed it and set my x min to 0 and max to 5, then i used the zero key to get around 2.383