Puzzle ZTKI 

Workings

Hint
How many 1x1 squares are there?

Answer
91 squares.

Reasoning
Squares of size:
   1 x 1 = 36
   2 x 2 = 25
   3 x 3 = 16
   4 x 4 = 9
   5 x 5 = 4
   6 x 6 = 1

Total = 36 + 25 + 16 + 9 + 4 + 1 = 91.