Since it's literally cut in half, that means the triangle's area is literally half of the rectangle's area. Therefore, if we write it as
the triangle's area (a)=(1/2)*(b) (the rectangle's area) we can multiply 2 to both sides to get 2*a=b. Since a is (2x+10), we can plug it in and use the distributive property (since we just can't add up variables and integers) to get
2*(2x+10)=4x+20=b
