Equivalent Characteristic Impedance for Different Trace Widths
Assuming the height above the ground plane is the same for the whole trace, the 5-mil-wide segments will have one characteristic impedance and the 10-mil-wide segment will have a different, lower, characteristic impedance.

The whole trace would not normally be thought of as having a single "equivalent impedance" characteristic. 

What you could do is calculate the input impedance, or the S-parameters of the trace, as a function of frequency. To do this, you could use a simulation tool like Keysight ADS, or Smith chart techniques (if you are only interested in the input reflections), or simply sit down and do a bunch of algebra.