Linear vs. Nonlinear Functions - Definition & Examples

Brenda has been working at the same company for $15$ years and decides to track her income received over that period of time. She finds that every three years, her pay is raised $3,600$ dollars/year, and her starting income was $42,000$ dollars/year. Would this trend yield a linear graph? If this trend continues, what would her income be after $21$ years?