This doesn't make sense provide more info? :)
Step-by-step explanation:

90 ones would make 9 tens. It makes 9 tens because theres a 0 after 90. The 0 represents another place value. You can write this as 9 groups of ten
L=W+12
2L+2W=48
2(W+12)+2W=48
2W+24+2W=48
4W=48-24
4W=24
W=24/4
W=6 ANS. FOR THE WIDTH.
L=6+12=18 ANS. FOR THE LENGTH.
PROOF:
2*18+2*6=48
36+12=48
48=48
Hope this helps:)