check the picture below.
bearing in mind that 1 foot is 12 inches, therefore, 1'6" is just 18", so the altitude is really 18 inches.
let's bear in mind that an isosceles triangle, has two twin sides, namely the ones coming from the top(vertex), so we can just find one side to get both, as you see there in the picture.
so the sides are, the base is 48 inches, and the twins are each 30 inches, so the perimeter of the triangle will be 48+30+30 = 108 inches, or 9 feet.
