ONE THE PROPERTIES OF A RHOMBUS STATES THAT ALL SIDES ARE EQUAL MEANING IF AD IS 39cm THRN BC IS ALSO BC SINCE AD=BC
IF SO THEN IN TRIANGLE BEC WE CAN USE THE THEOREM OF PYTHAGORAS TO FIND EC SINCE WE HAVE THE HYPOTENUSE EQUAL TO 39cm AND THE ADJACENT SIDE EB EQUAL TO 15cm
EC=36 cm
HOPE THIS HELPS