We can use the Pythagorean Theorem (a^2+b^2=c^2) to find the side lengths.
15^2=12^2+x^2
225=144+x
81=x
Which gives us that x=9 (you have to remember to find the square root)
Perimeter is 2l+2w, so 2(12)+2(9) = 42
Perimeter = 42
As for the area, we just multiply 9 by 12 to get us 108
Area = 108
:)