Q.Solve graphically : $x \ge 0$

A linear inequation in two variables $x,y$ is an expression such as $ax+by<c$, $ax+by\le c$, $ax+by>c$ or $ax+by\ge c$, where $a,b,c$ are real numbers and $a,b$ are not both zero. Every linear equation $ax+by=c$ is a straight line that splits the whole coordinate plane into exactly two open half-planes (points strictly on one side or the other) plus the line itself as their common boundary. Graphically solving a linear inequation means shading the correct one of these two half-planes: first draw the boundary line (solid if the inequality includes equality, i.e. $\le$ or $\ge$), then pick a convenient test point NOT on the line — the origin $(0,0)$ whenever the line doesn't pass through it, otherwise any other off-line point such as $(1,0)$ or $(0,1)$ — and substitute it into the inequation. If the test point satisfies the inequality, shade the half-plane containing that point (usually called the 'origin side' when the origin was used); if not, shade the opposite half-plane (the 'non-origin side'). This same idea covers the simplest cases too: $x\ge0$ shades everything on/right of the Y-axis, $y\ge0$ shades everything on/above the X-axis, and combining two such conditions (e.g. $x\ge0$ and $y\ge0$ together) picks out one whole quadrant. When two or more linear inequations must hold simultaneously (a 'system'), each boundary line is drawn and each half-plane shaded separately, and the region common to ALL of them (the overlap) is the graphical solution of the system.