Answer:
3x² + 13x + 12 ft²
Explanation:
To work out the area of a rectangle you have to multiply the length and width together. In this case the length is 3x + 4 and the width is x + 3
So
( 3x + 4 ) × ( x + 3 )
Remember FOIL when expanding brackets
F = First two so 3x × x = 3x²
O = Outer two so 3x × 3 = 9x
I = Inner two so 4 × x = 4x
L = Last two so 4 × 3 = 12
but we aren't finished
3x² + 9x + 4x + 12 = 3x² + 13x + 12
Don't forget unit! ft²