Final answer:
To solve the system by substitution, you need to substitute x - 4 for y in equation 2 which results in 6x - 4 = 20. The missing term in the blank is D. x + 4.
Step-by-step explanation:
To solve the system of equations by substitution, we need to substitute the value of y in equation 2 with the expression x - 4. The original equation 2 is 5x + y = 20. When we substitute x - 4 for y, the equation becomes 5x + (x - 4) = 20. Simplifying this equation gives us 6x - 4 = 20. To find the missing term that should replace the blank, we isolate the term with x by adding 4 to both sides of the equation, resulting in 6x = 24. Dividing both sides by 6, we get x = 4. Therefore, the missing term is D. x + 4.