Across period 3, the atomic radii from sodium to chlorine are 186, 160, 143, 117, 110, 104 and 99…

Step 1. Total fall. $186 - 99 = 87$ pm over $6$ steps.

Step 2. Average. $\dfrac{87}{6} = 14.5$ pm for each element.

Step 3. Biggest step. The changes are $-26$, $-17$, $-26$, $-7$, $-6$ and $-5$ pm. The first $-26$, from sodium to magnesium, comes first; silicon after aluminium also falls by $26$ pm.