15 Part A
You use the FOIL rule to get (x+6)(x+19) = x^2 + 25x + 114. So it appears you're a bit on the right track, but you forgot to multiply 6 and 19 to get 114.
That polynomial represents the area of the pool plus the area of the deck around the pool. The area of the pool only is x(x+13) = x^2 + 13x.
Subtract the two polynomials to get
(x^2 + 25x + 114) - (x^2 + 13x)
x^2 + 25x + 114 - x^2 - 13x
(x^2 - x^2) + (25x - 13x) + 114
0x^2 + 12x + 114
0 + 12x + 114
12x + 114
Therefore, the area of the deck only is represented by the expression 12x+114
Answer to part A: 12x + 114
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15 Part B
x = width of the pool
Plug x = 13 into the expression we found for part A
area of deck only = 12x + 114
area of deck only = 12*13 + 114
area of deck only = 156 + 114
area of deck only = 270
Answer to part B: 270 square feet