K=3gy/5f solve for g

I have a hunch you meant:
K=3gy/(5f)

based on that hunch ....
5fk = 3gy
g = 5fk/(3y)

those brackets are critical.

How did you do it?

To handle this type of equation you should be familiar with the steps to solve basic equations.
K=3gy/(5f)
multiply both sides by 5f

5f*K=3gy/(5f)*5f
5fk = 3gy
divide both sides by 3y
5fk/(3y) = 3gy/(3y)
5fk/(3y) = g
as shown above