x is a matrix in which the first column is the coefficients that multiply x, and the second are the coefficients on the right side of the equality.
y is a matrix in which the first column is composed by the coefficients on the right side of the equality and the second is the coefficients that multiply y.
M is a matrix in which the first column is the coefficients that multiply x, and the second is the coefficients that multiply y.
You take out the biggest factor of 9 and 36 which is 9c^2. The factors both have the c variable in common so you take the least exponent and put it with the gcf.