Find inc/dec interval of the following function:

f(x) = x^4-50x^2+4

My work:

f'(x) = 4x^3-100x

4x^3-100x = 0

4x(x^2-25) = 0

4x = 0 ----> x= 0

x^2-25 = 0----> x = 5, x = -5

So, (-5,0,5) are the critical point....

I cannot find the increasing and decreasing intervals, because when I substittue critical points in f(x) to positive or negative, it just give me negative value.

Can you plz help me by giving me the correct answer.

Thx