Multiplying it out using the distributive property, you have ...
... x^4 -(x^4 -7x^2 +7x^2 -49)
... = x^4 -x^4 +7x^2 -7x^2 +49 . . . . distribute the minus sign
... = x^4(1 -1) +x^2(7 -7) +49 . . . . . . collect like terms
... = 0 +0 + 49 . . . . . . . . . . . . . . . . . .simplify
... = 49
