as f(x) is an even function,as we know that integral of an even function between limits of positive and negative sign of same number(let say n) is the 2*integral of f(x) between limits 0 and n.
take f(x) as X and X^2.
For f(x)= X^2 integration will give positive value. (even function as (-1)^2 =1)
For f(x)= X integration will give 0. (odd function as X= -1 will give -1.