How do you decide how many solutions a linear system in three variables has?

Answer 1

Put the coefficients of the variables into a 3x3 matrix, and take the determinant of the matrix. If the determinant is not zero, then there is one solution. If the determinant is zero, then there are infinite solutions or there is no solution.

Think of a system of 2 variables, for simplicity. You have 2 equations and 2 variables (x & y). Each equation can be graphed as a straight line, hence the name 'linear system'. If the 2 lines are not parallel, then there will be only one point where they intersect, which is the one solution to the linear system. If they are parallel, then there is no solution(they never intersect), and if the two lines coincide, then infinite solutions(they intersect at every point). In both of the latter cases, the related matrix will have a determinant of zero.