Ambrose has indifference curves with the equation x subscript italic $2$ italic space italic equals italic space c o n s t a n t italic space italic minus italic space italic $4$ x subscript italic $1$ superscript italic $1$ italic divided by italic $2$ end superscript italic space italic space, where larger constants correspond to higher indifference curves. If good $1$ is drawn on the horizontal axis and good $2$ on the vertical axis, what is the slope of Ambrose's indifference curve when his consumption bundle is $(16,5)?$