1) In order to determine the perimeter of the figure, you take into account tht the perimeter is the sum of the values of all sides of a figure.
In this case, the sides of the triangle are s-4, s-8, and 6 respectivelly.
The perimer is the sum of the values of the sides, then, you have:
P = (s-4)+(s-8)+(6) "P is the perimeters"
= s-4+s-8+6 "sum simmilar terms, terms with variables and idependet term"
= 2s-6
Hence, the perimeter of the triangle is 2s - 6
2) The perimeter of a rectangle is P = 2w + 2l, in order to solve for w you proceed as follow:
P = 2w + 2l "subtract 2l both sides"
P - 2l = 2w "divide by 2 both sides"
(P - 2l)/2 = w
Hence, w = (P - 2l)/2