The equation is linear in two variables, x and y. It can't be solved, as such, for particular values of x and y, but a straight line graph represents the dependency of one variable on the other. The graph is a mapping of the function in which the range of values of x and y is the set of all real numbers, so the graph maps x values onto y and y onto x in a unique way: there is only one value of x for a particular value of y, and only one value of y for a particular value of x. There is a real value of x for every real value of y and vice versa, subject to the dependency function y=3x+4, and no real values are excluded. The ordered pair (x,y) forms the set of real numbers satisfying the equation. Every point on the line (and there are an infinite number of them) is an ordered pair, and forms an infinite set. Each ordered pair is effectively a solution to the equation, so there are an infinite number of solutions.