The need to find the value of r.
The given figure is a rectangle and we can find the perimeter when we sum all its sides.
P = r ft +r ft +3ft + 3ft
and the sum has to be equal to 18ft, then:
r ft +r ft +3ft + 3ft = 18 ft
Solve the equation for r to find it value:
r ft +r ft +3ft + 3ft = 18ft
2r ft + 6 ft = 18 ft
Subtract 6ft on both sides
2r ft + 6ft - 6ft = 18ft -6ft
2r ft =12ft
Divide by 2 on both sides
2r/2 ft = 12/2 ft
r = 6 ft
So 6ft its the value of r
Replace this value on the equation to confirm the result:
r ft +r ft +3ft + 3ft = 18ft
6 ft + 6ft + 3ft + 3ft = 18ft
12ft + 6ft = 18ft
18 ft = 18ft
The result is correct.