Could you explain to me decartes Rule of signs?

Question

Here are my questions:

Use Decartes Rule of signs to determine the possible number of postive and negative real zeros for the given function.

1. f(x)=-7x^9+x^5-x^2+6

choices are:
a. 2 or 0 positive zeros,2 or 0 negative zeros
b.2 or 0 positive zeros, 3 or 1 negative zeros
c.3 or 1 positive zeros, 3 or 1 negative zeros
d. 3 or 1 positive zeros, 2 or 0 negative zeros

2. f(x)10x^3-8x^2+x+5
my choices are:
a. 3 or 1 positive zeros, 1 negative zero
b.2 or 0 positive zeros, no negative zeros
c.3 or 1 positive zeros, 2 or 0 negative zeros
d.2 or 0 positive zeros, 1 negative zero

Answer 1

I will be happy to check your answers or thinking.

http://www.purplemath.com/modules/drofsign.htm