(a²—16b²/4ab) ÷ (1/4b—1/a) = ((a—4b)(a + 4b)/4ab) ÷ (a/4ab—4b/4ab) = ((a—4b)(a + 4b)/4ab) ÷ ((a—4b)/4ab) = ((a—4b)(a + 4b)/4ab) · (4ab/(a—4b)) = (a—4b)(a + 4b)4ab/4ab(a—4b) = a + 4b = (3+5/13) + 4(4+2/13) = 44/13 + (4 · 54/13) = 44/13 + 216/13 = (44 + 216)/13 = 260/13 = 20