cv::Mat element matrix multiplication

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I need to take concrete element from three channel matrix:

a.row(0).col(0)

and use this element for multiplication:

multiply(a.row(0).col(0), b, c);

Something like this:

Mat a(5, 5, CV_32FC3);  
Mat b(5, 5, CV_32FC3); 
Mat c;

multiply(a.row(0).col(0), b, c);

But it is not working. Do you have some tip how to do it?

P.S. : it is really easy for one channel image using

a.at<float>(0,0)

Edit

Ok, let me give an example.

Mat a:

[ 91,   2,  79, 179,  52, 205, 236,   8, 181, 239,  26, 248, 207, 218,  45;
 183, 158, 101, 102,  18, 118,  68, 210, 139, 198, 207, 211, 181, 162, 197;
 191, 196,  40,   7, 243, 230,  45,   6,  48, 173, 242, 125, 175,  90,  63;
  90,  22, 112, 221, 167, 224, 113, 208, 123, 214,  35, 229,   6, 143, 138;
  98,  81, 118, 187, 167, 140, 218, 178,  23,  43, 133, 154, 150,  76, 101]

Mat b:

[  8,  38, 238,  84,  47,   7, 117, 246, 163, 237,  69, 129,  60, 101,  41;
 190,  50,  90,  72, 168, 109, 121, 220, 114, 248,  99, 202, 199, 212,  79;
 128, 198,  90, 168,  76, 145, 181, 118,   8,  63, 114, 217, 164, 158, 217;
 237, 147,  44, 207,  54, 182,  65, 197, 191, 239,  72, 166, 236, 240,   3;
 151,  91, 246, 116, 238,  94,  63, 252, 232,  25,  17,   8, 166, 116,  81]

I want to take element [0; 0] from the a which is (91, 2, 79) and use it for per-element multiplication to achieve Mat c:

[  728, 76, 18802, 7644, 94, 553, 10647, 492, 12877, 21567, 138, 10191, 5460, 202, 3239;
 17290, 100, 7110, 6552, 336, 8611, 11011, 440, 9006, 22568, 198, 15958, 18109, 424, 6241;
 11648, 396, 7110, 15288, 152, 11455, 16471, 236, 632, 5733, 228, 17143, 14924, 316, 17143;
 21567, 294, 3476, 18837, 108, 14378, 5915, 394, 15089, 21749, 144, 13114, 21476, 480, 237;
 13741, 182, 19434, 10556, 476, 7426, 5733, 504, 18328, 2275, 34, 632, 15106, 232, 6399]