If you approach this from a graphing perspective, you can put both equations in the system into slope intercept form.
System A:
Line 1: 3x + 5y = 8
5y = -3x + 8
y = -3/5 x + 8/5
Line 2: 3x + 5y = 7
5y = -3x + 7
y = -3/5 x + 7/5
Because both lines have the same slope, but different y-intercepts, the lines are parallel and will never intersect. This is why there is no solution to the system.
System B:
Line 1: y = 7x is in slope intercept form.
Line 1: y = 3x is in slope intercept form.
Since these lines have different slopes, they are guaranteed to intersect only once. There is a single solution.
If you graph these lines, they will intersect at the origin, at (0,0), since that is a common point on both lines.