I think you need an extra set of curly braces around your variables to fix the formatting.
I’m not sure if you want this or not, but if you prefer the negative sign in front of the fraction, you can create a conditional.
num = -7
den = 4
sign = when num<0 "-" otherwise ""
absNum = numericValue("\operatorname{abs}(${num})")
content: "Find the equation of a line that passes through the point `(8, 2)` and is parallel to the line `y=${sign}\frac{${absNum}}{${den}}x+2`"