Integral of $$$3 \cos{\left(x \right)}$$$

Apply the constant multiple rule $$$\int c f{\left(x \right)}\, dx = c \int f{\left(x \right)}\, dx$$$ with $$$c=3$$$ and $$$f{\left(x \right)} = \cos{\left(x \right)}$$$:

$${\color{red}{\int{3 \cos{\left(x \right)} d x}}} = {\color{red}{\left(3 \int{\cos{\left(x \right)} d x}\right)}}$$

The integral of the cosine is $$$\int{\cos{\left(x \right)} d x} = \sin{\left(x \right)}$$$:

$$3 {\color{red}{\int{\cos{\left(x \right)} d x}}} = 3 {\color{red}{\sin{\left(x \right)}}}$$

Therefore,

$$\int{3 \cos{\left(x \right)} d x} = 3 \sin{\left(x \right)}$$

Add the constant of integration:

$$\int{3 \cos{\left(x \right)} d x} = 3 \sin{\left(x \right)}+C$$

$$$\int 3 \cos{\left(x \right)}\, dx = 3 \sin{\left(x \right)} + C$$$A