We know that the perimeter formula for a triangle would be P=s1+s2+s3 (s1=first dies, s2=second side, s3=third side)
Since we know the perimeter and two sides, we can plug these in a solve for the last side
60=23+17+s3
60=40+s3 (combine like terms)
20=s3 (subtract 40 from both sides)
So then side c would be equal to 20 m
Hope this helps :)