As we know that all the sides of a square are of the same measure and also all the interior angles of a square are of 90° measure

(1) A rough sketch of this square:

(a) Draw a line segment RE of 5.1 cm and an angle of 90° at point R and E

(b) As vertex A and D are 5.1 cm away from vertex E and R respectively therefore cut line segments EA and RD, each of 5.1 cm

(c) Join D to A

READ is the required square