What are the two ways of invoking methods?

When a method returns the value, the entire method call can be assigned to a variable. Do you agree with the statement?

Debug the errors and rewrite the following method prototypes:

(a) int sum(x,y);

(b) float product(a,int y);

(c) float operate(int x, float=3.4);

(d) float sum(int x,y);

Write down the main method which calls the following method:

int square(int a)
{
    return(a*a);
}