I need help :((((((((

An equation has an infinite number of solutions if the left and right sides are identical. Let's simplify them both and then decide the value of ?.
Left hand side:
[tex] 5(x-4)+? = 5x - 20 + ? [/tex]
Right hand side:
[tex] 1+12x-21 = 12x-20 [/tex]
So, both sides have a numeric part of -20. We can use the "?" value to make sure that the literal part is the same as well. So, we want
[tex] 5x+? = 12x \iff ? = 12x-5x = 7x [/tex]