r=2s-6t+5/2 solve for s

$R = 2s - 6t + 5/2$

We subtract both sides by 5/2 to get

$R - 5/2 = 2s - 6t$

We add $6t$

to both sides and get

$R - 5/2 + 6t = 2s$

Dividing both sides by gets you

$\frac{R + 6t - 5/2}{2} = s$

Hope this helps!