Solution

Note INTENT is not essential as it may not have been covered in the course so far.

External procedure version:

     REAL FUNCTON Summy1(x,y)
      IMPLICIT NONE
      REAL, INTENT(IN) :: x, y
       Summy1 = x+y
     END FUNCTON Summy1

    PROGRAM Main
     IMPLICIT NONE

      PRINT*, Summy1(1.0,2.0)
      PRINT*, Summy1(1.0,-1.0)
      PRINT*, Summy1(0.0,0.0)
      PRINT*, Summy1(1.0E54,9192652.0)

    END PROGRAM Main

Internal procedure version,

    PROGRAM Main
     IMPLICIT NONE

      PRINT*, Summy1(1.0,2.0)
      PRINT*, Summy1(1.0,-1.0)
      PRINT*, Summy1(0.0,0.0)
      PRINT*, Summy1(1.0E54,9192652.0)

    CONTAINS

     REAL FUNCTON Summy1(x,y)
      REAL, INTENT(IN) :: x, y
       Summy1 = x+y
     END FUNCTON Summy1
    END PROGRAM Main