What is the centre of mass?

If the linear density of a rod of length $3 \ m$ varies as $\lambda = 2 + x$,then the position of the centre of mass of the rod is

Four particles of masses $m, 2m, 3m$ and $4m$ are arranged at the corners of a parallelogram with each side equal to $a$ and one of the angles between two adjacent sides is $60^o$. The parallelogram lies in the $x-y$ plane with mass $m$ at the origin and $4m$ on the $x$-axis. The centre of mass of the arrangement will be located at

$A$ uniform disc of radius $R$ is placed over another uniform disc of radius $2R$ made of the same material and having the same thickness. The peripheries of the two discs touch each other. Locate the centre of mass of the system,taking the center of the large disc at the origin.

Consider the following statements regarding the Centre of Mass $(CM)$ of objects of radius $R$ from their geometric centre:
$[1]$ $CM$ of a uniform semicircular disc is at $2R/\pi$.
$[2]$ $CM$ of a uniform semicircular ring is at $4R/3\pi$.
$[3]$ $CM$ of a solid hemisphere is at $4R/3\pi$.
$[4]$ $CM$ of a hemispherical shell is at $R/2$.
Which of these statements are correct?

$A$ uniform square wooden sheet of side $a$ has its centre of mass located at point $O$ as shown in the figure below on the left. $A$ square portion of side $b$ of this sheet is cut out to produce an $L$-shaped sheet as shown in the figure on the right. The centre of mass of the $L$-shaped sheet lies at the point $P$ (in the above diagram),when