Yes
Angle 1 and angle 4 add up to 180 degrees because they are on the same straight line split into two parts, and a straight line is 180 degrees. Angle 4 and angle 7 are corresponding angles, which means they occupy the same relative position at each intersection where a straight line crosses two others. If the two lines are parallel, the corresponding angles are equal. Supplementary angles mean that any two angles add up to 180 degrees. Since 4 and 7 are corresponding, 1 and 7 must be supplementary
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
