What is its perimeter of a bedroom that is 10 ft by 11 ft. ?

Draw it on paper, 10 by 11. That is, it's a rectangle. So the side opposite to the 10ft length must also be 10. Same for the 11ft length.
..........11ft
...... _______
10ft |............| 10ft
......|_______|
..........11ft

Then just add them up = 42ft.

= 2(10 + 11)
= 2(21)
= 42

Answer: 42 feet

Another way:
= 2(10) + 2(11)
= 20 + 22
= 42

42

add the measurement of the sides. that'd be 42 ft.

42 ft

42 Linear feet (110 square feet).

10+10+11+11= B)42

2 (10 + 11) ft
2 x 21 ft
42 ft

add 10 ft + 10 ft + 11 ft + 11 ft = 42 ft.

2(10 ft + 11 ft)
= 2(21 ft)
= 42 ft