From the figures, notice that the number of dots increases by 7. This give us the following sequence:
To obtain the 2nd step, we add one 7 to the first term.
To obtain the 3rd step, we add two 7s to the first term.
To obtain the 4th step, we add three 7s to the first term.
Notice that we multiply the 7 by the difference of the nth step and 1.
Therefore, to find the 27th step, we add 26 7s to the first term.
Therefore, there must be 189 dots in the 27th step.