If you mean x4-ax3+a2x2-25a4
Applying synthetic division:
-2a | 1 -a a2 0 -25a4
1 -2a 6a2 -14a3 | 28a4
1 -3a 7a2 -14a3 | 3a4=243 (remainder),
a4=81, a4-81=0=(a2-9)(a2+9)=(a-3)(a+3)(a2+9).
Therefore a=±3.
The polynomial is x4-3x3+9x2-2025 or x4+3x3+9x2-2025.