The perimeter would be found by adding the round side of the semicircle and the other three sides of the rectangle.
Without a picture I am imaging that the semicircle's straight side is either the length or height of the rectangle. That number would be the diameter of the semicircle.
When it comes to circles, the perimeter is called a circumference. The rounded portion of the semicircle is half the circumference of a circle. Basically find the circumference then divide that number by two..
Circumference of a Circle
C=2πr or C=πd