1) f(x)= x4+5x3-x2-5x
Statements 1, 3, and 4 are correct. 2 is not because f(5) does not equal 0.
The Rational Root Theorem is:\
Let f(x) be a polynomial with integral coefficients. The only possible rational zeros of f(x) are:
\(\frac{p}{q}\)
where p is a divisor of the constant term and q is a divisor of the leading coefficient.
So, fators of the constant on top and factors of the LC on bottom.
\(\frac{\pm1, \pm2, \pm4}{\pm1,\pm2}\)
Now, pair them together.
Divide all of the top factors by 1 first, then 2.
\(\pm1,\pm2,\pm4,\pm \frac{1}{2}, \pm1,\pm2\)
Repeating factors can be dropped so the possible factors are \(\pm1,\pm2,\pm4,\pm \frac{1}{2}\) or \((x+1),(x-1),(x+2),(x-2),(x+4),(x-4),(2x+1),(2x-1)\).
So your possible factors are correct.
To see which ones are factors, I woud graph it.
This graph shows that \((-2,0), (-\frac{1}{2},0), (2,0)\) are the zeros. (Here is the graph: https://www.desmos.com/calculator/fx9umcmzu0)
As factors those are (x+2), (2x+1), and (x-2).