How would you get the equation to figure the area out?

1.(Rectangle) the length is 3x - 2 and the width is 4.





2.(Triangle) the height is 5a + 7 and the base is 18.





3. (Triangle) the height is 13 and the base is 6 - 2y





can you give me the equation to figure out the area. I looked in the back of the book to get the idea but, I also have to know how I got it. I already know the answer to the second one. It's (45a + 63) units 虏How would you get the equation to figure the area out?
Area of rectangle = Length * Width


Area of triangle = (1/2) * Base * Height





#2: Area = (1/2) * (5a+7) * 18 = 9*5a + 9*7 = 45a + 63 units^2