In MATLAB for entering matrix values, commas and spaces can be used (interchangeably) to separate values left to right (i.e. in the same row) and a semicolon can be used to separate the end of a row of values from the start of the next row.
a = [1 2 3, 4, 5; 11, 12, 13 14, 15] % 1 to 5 appear in the first (top) row and 11 to 15 in the second row.
% You should enter the same number of elements in each row, otherwise you'd get an error.
If you're new to using MATLAB, I would recommend the 'MATLAB OnRamp' course linked below. It covers the fundamentals of MATLAB well and would help new users a lot in picking up MATLAB.