Suppose 3x+7= integral from a to x (f(t)dt) What is the value of a?

I applied the fundamental theorem of calculus part 2 to get that f(x) = 3.

Then, I applied the first part of the fundamental theorem to get that the integral from a to x of 3 dt = 3x - 3a.

Looking at the solutions, it says that 3x - 3a = 3x + 7. Then you simply solve for a.

I don't understand why this relationship is true. How is 3x - 3a equal to 3x + 7?