The difference between $7$ and $3.4$ is $3.6$.

To determine the difference between two numbers, you subtract the smaller number from the larger one. In this instance, you subtract $3.4$ from $7$. Therefore, the calculation is expressed as:

When you carry out this subtraction, you arrive at the result of $3.6$.

This process is quite simple: align the decimal points and subtract as you would with whole numbers. If necessary, you can add zeros to facilitate the computation. For example, you can represent $7$ as $7.0$. Then, you perform the subtraction:

This calculation also yields the result of $3.6$.

Mastering the technique of finding the difference between numbers is a fundamental skill in mathematics. This skill is applicable in various real-life scenarios, such as calculating change, measuring distances, or comparing quantities. Regular practice of these basic arithmetic operations will enhance your overall mathematical proficiency and prepare you for more complex problems in the future.