The two 10's are wired in series. So that part is simple addition, if they are 8 ohms each then the two of them in series is 16ohms.
Now you have 16ohms wired in parallel with 8ohms (the 18).
The equation for parallel is product over sum. So (16x8) divided by (16+8)
Maybe your meter is reading high, disconnect one of the speakers and measure it by itself to get an idea of what your meter reads for an 8 ohm speaker. Also, DC resistance readings on a meter are usually lower than the impedance rating. So for a calculated 5.33ohm impedance, we expect the meter to read a little less.
Also note what your meter reads with the probes shorted together. This should be zero, if it is 1 or 2 ohms or something, you must subtract that from your measurements.