Sides of a parallelogram are $30$ cm and $26$ cm and the smaller diagonal is $28$ cm. The semi-perimeter of the triangle formed by the diagonal is —

The smaller diagonal splits the parallelogram into a triangle whose three sides are the two given sides and the diagonal: $30$, $26$, and $28$ cm. Its semi-perimeter is $s=\dfrac{30+26+28}{2}=42$ cm.