I will guess that you mean the "2" after the "x" to be an exponent, and that the square is only on the "x", not the log or the entire expression, so the expression is actually:
. . . . .1 - log<sub>7</sub>(x<sup>2</sup>)
However, without a value for "x", I see no way to evaluate or "simplify" this expression. One could, I suppose, expand it, but that's a different thing, and wouldn't probably be "simpler" than what you've got now.
Eliz.