A matrix is diagonal if
In other words, you want non-zero elements only when the indices of the row and the column are the same: the first element of the first row must be non zero, the second element of the second row must be non-zero, and so on. All other elements must be zero.
The first equation given
We have to add 1 and 5 to the right side first. We will get,
To get rid of 6 from the right side we have to subtract 6 from both sides.
To find n we have to move -2 to the other side by dividing both side by -2.
So we have got the required answer for the first question.
The solution is n = 0.
The second equation given,
First we have to move 7x to the left side by subtracting it from both sides.
Now we have to move -2 to the right side by adding 2 to both sides.
We have got the required answer for the second question.
The solution is x = -7.
The third equation given,
We have to get rid of that negative sign from both sides. As we have negative sign to both sides we can cancel it out. We will get,
Now we have to move 4 to left side by subtracting it from both sides.
So we have got the required answer .
The solution is x = 4.