unique solution means there is one intersection for the two lines. so there is a single x = ? and y = ? that solves the system. as i said, we have a unique solution when the two straight lines intersect

we have infinite solutions when the lines are, in fact, the same line. so one line lies on top of the other, so there are an infinite amount of intersecting points

we have no solution if the lines never intersect, that is, if the lines are parallel.

all you have to do, if figure out the values for a and b that cause the respective situations to happen