jmc

The slopes of perpendicular lines are negative reciprocals. Therefore, the slope of line $b$ is the negative reciprocal of $-2$, which is $\frac{1}{2}$. The slopes of parallel lines are the same, therefore the slope of line $a$ is also $\frac{1}{2}$. Using the point-slope formula, the equation of line $a$ is $y-2=\frac{1}{2}(x-1)$. Changing the form to slope-intercept, we get the equation $y=\frac{x}{2}+\frac{3}{2}$. Therefore, the y-intercept is $\boxed{\frac{3}{2}}$.