Working through the above problem, an instructional video tells me to solve row 1, column 1 of the matrices as follows;

2a+b - (2a - b)

The video then goes onto used the disributive property, assuming a negative "1" in front of the parentheses, thus "negative 1 multiplied by negative 2a"

Is this correct? The reason I am asking is because I thought it would be negative 1 multipled bypositive2a, as there isn't a sign within the parentheses?

The video I am watching solves the equation as follows;

2a + b - (2a - b)

= 2a + b - 2a + b

= 2a - 2a + b + b

= 2b

I can follow the rest of the solution, I am just confused as to why the teacher assumes a negative sign in front of the 2a which is within the parentheses.