2.4z+1.2z-6.5=0.7 I don't get it

Basically that answer is incorrect. If you'd learn about combining, multiplying, dividing or subtracting like terms, this will be a simple problem. To be specific, like term is a number with a letter with it.

For example, 2.4z is a like term.

so you can only add like term, not any other normal number. When two terms have the same "letter", you will have to do what the sign said so in the problem

2.4z + 1.2z - 6.5 = 0.7
3.6z - 6.5 = 0.7 (Add but leave the z alone because you are not multiplying)

The final answer is 3.6z - 6.5 because you cannot do anything else to that problem.

3.6z-6.5=0.7 is not true.