Find the equation of the regression line y=ax+b and estimate y when x=12, given a=3/5 and b=-2.

The general equation for the regression line of $y$ on $x$ is $y = ax + b$.
From the problem statement and the partial solution, we are given the gradient $a = \frac{3}{5}$ and we have calculated the y-intercept $b = -2$.
Substitute these values into the equation:

$$y = \frac{3}{5}x - 2$$

Substitute $x=12$ into the regression equation found in Step 1:

$$y = \frac{3}{5}(12) - 2$$ $$y = \frac{36}{5} - 2$$ $$y = 7.2 - 2$$ $$y = 5.2$$

The equation of the line is $\boxed{y = \frac{3}{5}x - 2}$ and the estimated value of $y$ when $x=12$ is $\boxed{5.2}$.