First, we invert the `(st)/4` term to give `4/(st)` and then multiply by that inverse:

`(sr^2)/(2t)-:(st)/(4)=(sr^2)/(2t)xx(4)/(st)`

Next, cancel out the two s's on the top and bottom; and cancel the 4 on top with the 2 on bottom to give 2 on top.

`=(r^2xx2)/t^2`

`=(2r^2)/t^2`

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