Explanation:
the area of a rectangle is always
length × width
in our case that means
(7x - 6) × (5x + 3)
now, when we do the multiplication (every term of one side multiplied by every term of the other side and all added up while considering the signs of the individual multiplication results) :
7x×5x + 7x×3 - 6×5x - 6×3 = 35x² + 21x - 30x - 18 =
= 35x² - 9x - 18