6) \( 21=-5+3(d-6) \quad d=14 \frac{3}{5} \)

Let's dive into the historical context of equations! The concept of solving equations has roots that date back to ancient civilizations like the Babylonians, who used geometric methods to solve linear equations around 2000 BC. Fast forward to the 17th century, when mathematicians like René Descartes and François Viète revolutionized algebra with symbolic notation, paving the way for the modern methods we use today. Now, onto the real-world application! Solving equations like the one you've provided is crucial in various fields such as engineering, finance, and physics. For instance, engineers compute load distributions on structures using linear equations, ensuring safety and stability. Meanwhile, finance professionals use them to model trends and make predictions based on current data, helping businesses thrive in competitive markets.