I’m assuming that u meant 135cm and NOT 135cm squared because radius is length
First find the circumference of the entire circle
= 2 x pi x radius
= 2 x pi x 135
= 848 cm
Next divide it by two because it is semi circle
=848 / 2
= 424cm
This length is only for the curved part of the semi circle. If u want the full length around it add it to the diameter which is twice the radius