Applying scalar multiplication to matrix
Braces aren't required as initialization will proceed by row.
However it is much easier to read if you initialized your
two-dimensional array like this:

int sampleMatrix[3][5] = {
    {7,16,55,13,12},
    {12,10,52,0,7},
    {-2,1,2,4,9}
}

Also note, it is not required that the entire array be initialized.
If you use inner pairs of braces you can force initialization to
whatever you want. Unspecified values are set to zero by default.

int sampleMatrix[3][5] = {
    {7,16,55,13},  //Last element in each row is zero ; test it yourself
    {12,10,52,0},
    {-2,1,2,4}
}

Use variables to represent the number of rows and columns for the
matrices. Magic numbers are bad. This will also allow you to pass
matrices of different dimensions.

void scalarMultiply(int nRows, int nCols, int matrix[nRows][nCols], int scalar);
void displayMatrix (int nRows, int nCols, int matrix[nRows][nCols]);


It is best practice to keep your function prototypes outside of
main(). Keeping it in main() makes it harder to find.
The outout layout looks a bit messy. Change your printf statement
to include a field width specifier.

     printf ("%5i", matrix[row][column]);


%5i tells printf to display integers and their sign that take up 5 columns.
Use proper spacing:

    scalarMultiply (sampleMatrix, 2);

    printf ("\nMultiplied by 2:\n");
    displayMatrix (sampleMatrix);

    scalarMultiply (sampleMatrix, -1);

    printf ("\nThen multiplied by -1:\n");  //Keep related functions together      
    displayMatrix (sampleMatrix);