Let's recompute the intercept $a$ using the correct formula:

$a = \frac{\sum y - b\sum x}{n}$

I will redo the calculation to ensure accuracy.The intercept $a$ was correctly calculated as approximately 15.75. This value is indeed correct based on the calculations. The trend equation remains:

$y = 15.75 + 1.86x$

If you have any further questions or need additional clarification, feel free to ask.