(07.01 lc) a set of numbers is shown below: {5.4, 6.2, 7.4, 9.2, 15.1} the number __ from the set will make the equation x + 3 = 8.4 true.

You could guess and check, but it would be easier to isolate x to find the answer.
To do this, just subtract 3 from both sides.
8.4 - 3 = 5.4
x + 3 - 3 = x
x = 5.4

So the number in the set that makes x + 3 = 8.4 true is x = 5.4.

Now check the
x + 3 = 8.4
5.4 + 3 = 8.4
8.4 = 8.4
This is true, so 5.4 is the correct answer.

Hope this helps!