Answer: the maximum is 25.
Explanation: a max/min can occur on the endpoints of a function and critical points of the function's derivative.
f(x)=x^4-x^2+13
f'(x)=4x^3-2x
The critical points of f'(x) occur when f'(x) is zero or undefined. f'(x) is not ever undefined in this case, so we just need to find the x values for when it's zero.
0=4x^3-2x
x=.707, -.707
Now that we have the critical points of f'(x) (.707 and -.707) and endpoints (-1 and 2), we can plug in these x values into the original function to determine its maximum. When you do this you'll find that the greatest y value produced occurs when x=2 and results in a max of 25.