To set up an equation for this you can use a variable to represent your unknown (in this case the unknown is how long it will take to frame 54 houses)
We know that 3 houses can be framed in 1 week so you can represent the equation as
3x=54 (3 being the amount of houses framed in 1 week, x being how many weeks it will take to frame 54 houses)
You can isolate X by dividing both sides by 3 (3x/3 = 54/3) which simplifies to
x = 54/3
