not unless it is equated to something where you can distribute the minus on the other side as well:
so, say you had: 21 - 4x - x^2 = 0
then it is okay to multiply both sides by -1 to get: x^2 + 4x - 21 = 0
but if you just have the expression by itself, you cannot do that, the best you can do is to say:
leave the minus outside of the brackets