PLEASE PLEASE HELP. I’ve wasted so many points on this problem :(

Answer:
13 cm
Step-by-step explanation:
A^2+12^2=15^2
A^2+144=225
A^2=81
A=9
9x2=18
31-18=13 cm
Find the perimeter of a rectangle whose width is 3 cm and whose diagonal is 3 √5 cm long.
A^2+3^2=3√5^2
A^2+9=45
A^2=36
A=6
3+3+6+6=18 cm