How to find the unknown dimension of a triangle? (see problem in picture below)

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 :)