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the functions y=sin(x) and y=cos(x) are the standart functions; it means, the period of each is 2Ï€, maximum value is '1', minimum value is '-1'.
Using the properties described above, to find the max. value for the function y=2cos(x): 2*1=2, where 2- the amplitude of the given function, 1- maximum of the standart function;
the max. value for the function y=3sin(x+Ï€): 3*1=3, where 3 - the amplitude of the given function, 1- maximum of the standart function.
For more details see the attached picture.
In other words, common equation for such functions is y=A*sin(ωx+Ф), where A- amplitude, ω - frequency and Ф - initial phase of the given function.
Max. and min. values depend on A, all the points of the function repeat every 2Ï€, including max. and min. values.

Answer with explanation:
- The function f(x) is given by:
[tex]f(x)=2\cos x[/tex]
We know that the function f(x) is a cosine function and the maximum value is obtained by the function when this cosine function takes the maximum value.
We know that:
[tex]-1\leq \cos x\leq 1[/tex]
This means that the cosine function  takes the maximum values as: 1
and when [tex]\cos x=1[/tex] then
[tex]f(x)=2\times 1\\\\i.e.\\\\f(x)=2[/tex]
i.e. Maximum value of function f(x)=2
- The function g(x) is given by:
[tex]g(x)=3\sin (x+\pi)[/tex]
Again the function g(x) will attain the maximum value when the function sine will takes the maximum value.
We know that the range of the sine function is: [-1,1]
This means that the maximum value of sine function is: 1
Hence, when [tex]\sin (x+\pi)=1[/tex] then,
[tex]g(x)=3\times 1\\\\i.e.\\\\g(x)=3[/tex]
i.e. Maximum value of function g(x)=3
     The function g(x) has the largest maximum value.
              ( Since 3>2)