How would write an equation of a regression line?

The equation for a regression line is standarly written like this: 

y = a + bx + e

y is the letter we usually use to denote that this is a (regression) equation we're talking about

a is the intercept in this case. a tells you what at what value of y would the line start/cross with the y-axis if our x was 0. 

b is the slope. It tells you how steep (or not) would our regression line be. 

e is usually also added but not necessarilly needed. e is used in statistics and tells us what amount of error did our regression line get (because we usually use regression lines to predict data).