1. **Stating the problem:** We have a straight line labeled $(D_1)$ passing through three points $A_x$, $B_x$, and $C_x$ arranged from bottom-left to top-right. 2. **Understanding the line and points:** Since the points lie on the same straight line, they are collinear. The line $(D_1)$ can be described by a linear equation of the form $$y = mx + b$$ where $m$ is the slope and $b$ is the y-intercept. 3. **Finding the slope:** The slope $m$ between any two points $(x_1, y_1)$ and $(x_2, y_2)$ on the line is given by $$m = \frac{y_2 - y_1}{x_2 - x_1}$$. 4. **Using the points:** If coordinates of $A_x$, $B_x$, and $C_x$ are known, calculate the slope using any two points. Since the points are collinear, the slope between $A_x$ and $B_x$ will be the same as between $B_x$ and $C_x$. 5. **Equation of the line:** Once $m$ is found, use one point, say $A_x = (x_0, y_0)$, to find $b$ by substituting into $$y_0 = mx_0 + b$$ which gives $$b = y_0 - mx_0$$. 6. **Final line equation:** Substitute $m$ and $b$ back into $$y = mx + b$$ to get the equation of $(D_1)$. 7. **Summary:** The key steps are to find the slope using the points, then find the y-intercept, and write the line equation. This describes the straight line $(D_1)$ passing through $A_x$, $B_x$, and $C_x$.