Answer:
See explanation below
Explanation:
A system of linear equations is a system that is formed when you have two or more variables, for example:

In this case, we have 3 equations with 3 variables in total (x, y and z). Thus, we would need to solve a system of linear equations (they are linear because none of the variables have exponents).
On the other side, a single linear equation has only one variable that we need to find, for example:

In this case we would just have to solve for x

Thus, we need to solve a system of linear equations when we have more than one variable (and more than one equations) as opposed to when we have just one variable and a single equation (single linear equation)