I know how to plot indifference curves; simply take the utility function and plot some level curves in $2D$.
But how to plot a specific indifference curve, so all bundles on it are indifferent to a specific $(a,b)$ bundle?
Would I calculate the utility for $(a,b)$, then set $U(x,y)$ equal to that utility, then plot that level curve?
This is a good idea.
Suppose $U(x,y)=x^{1/2}\cdot y^{1/2}$ and $(a,b)=(36,100)$. Thus $U(36,100)=6 \cdot 10=60$
$60=x^{1/2}\cdot y^{1/2} \Rightarrow y=\frac{3600}{x}$