Decomposing Changes in ROA into Changes in Operating Margin and Asset Turnover via Total Factor Productivity (TFP)
The formula for OM is:

$$ OM = 1 - \frac{1}{TFP\times PCR}$$

In the baseline year, you have that Operative Margin (OM) = 0.247. Further, by assumption, price change ratio (PCR) = 1. Then:

$$TFP = \frac{1}{1-OM} = \frac{1}{1-0.247}=1.328$$

Then, you are told that the TFP changed by 2.6%. Therefore, the new value for TFP is 1.3625 (equivalent to 1.328 $\times$ 1.026). With this new TFP, and assuming that PCR = 1, you can find the new value for OM:

$$ OM = 1 - \frac{1}{1.3625\times 1}$$
$$ OM = 1 - 0.734$$
$$ OM = 0.266$$

There you get the 26.6% result.