For a system of 2 equations in 2 unknowns, there are 3 cases:
slopes are different — one solution
slopes are the same and y-intercepts are different — no solutions
slopes and y-intercepts are the same — infinitely many solutions
When slopes are different, the two lines intersect at <em>one</em> point, the <em>solution</em>.
When slopes are the same, the lines may be either parallel (different y-intercepts) or the same (same y-intercepts). If the lines are parallel, there are no points of intersection, hence <em>no solutions</em>. If the lines are the same line, they intersect at all points, so there are <em>infinitely many solutions</em>.
True. using the first and second terms you can easily find the common difference by subtracting the first term from the second term. When you know the common difference you can then use it to find the other terms of the arithmetic sequence.