$\"\"$

For adding matrices, their dimension should be equal.

First write the dimension of the matrix.

The dimensions of matrix C and D are $\"\"$ .

The operation C + 3D is possible. $\"\"$

Multiply the matrix D by 3.

$\"\"$

$\"\"$ (Multiply each element by 3)

$\"\"$ (Simplify) $\"\"$

Add the matrix C + 3D.

$\"\"$ (Substitute the values of C and 3D) \ \

$\"\"$ (Add corresponding elements)

$\"\"$ (Simplify) $\"\"$

The value of C + 3D is $\"\"$ .