opamp voltage offset balance resistor for varying input Z
But my input terminal(+) has an impedance which will vary according to
  the input frequency, now I cant choose the fixed balancing resistor,
  how to solve this problem


If your input source has a DC impedance then this is subtracted from the 7.5 kohm and what remains becomes Rb.

If your input source is capacitively coupled then you need to use a resistor from +in to ground and this would be 7.5 kohm in the example.

Alternatively, choose an op-amp with much lower input bias currents. You'll still need a resistor to ground though if capacitively coupling.

The bigger picture is that the op-amp will have an input offset voltage and this can only be countered by Vos nulling. So, if you don't use a Vos nulling circuit, there's no point worrying about resistance matching if the bias current induced error voltage is significantly less than the op-amp's inherent input offset voltage.