If the triangles are congruent, then their corresponding parts are also congruent.
First check to make sure the orientation of the triangles is correct. This is so we can easily compare corresponding parts.
Angle d is 180-90-52 = 38 degrees (sum of interior angles of triangle is 180 degrees), and this is the same as the angle on the triangle on the left.
Therefore the side with a length of 3 units is congruent to side c, which means side c is 3 units.