How do I manually multiply and add to a 128-bit number by a 64-bit number

I have a 128-bit uint stored as two 64-bit uints and I need to add and multiply the latter to the former but I'm not really sure how to get started on this. Do I need to break the 128-bit number up into four 32-bit numbers and the 64-bit number into two 32-bit numbers or is there a quicker way to do this?

void                                                                            
_add(                                                                           
    uint64_t x[2],                                                                
    uint64_t y                                                                    
) {          
    x[0] += y;
    x[1] = ?                                
}     

void                                                                            
_multiply(                                                                      
    uint64_t x[2],                                                                
    uint64_t y                                                                    
) {          
    x[0] *= y;
    x[1] = ?                                                       
}                                                                               
                                                                                

Read more here: https://stackoverflow.com/questions/66322781/how-do-i-manually-multiply-and-add-to-a-128-bit-number-by-a-64-bit-number