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In the expression $3a + 4$, the symbol $a$ represents a variable or an unknown value.

In mathematics, a variable is a symbol that signifies a number which can change or vary. In the expression $3a + 4$, $a$ serves as the variable. The number $3$ in front of $a$ is known as the coefficient; it indicates that $a$ is multiplied by $3$. The number $4$ is classified as a constant, meaning it remains unchanged regardless of the value of $a$.

When you encounter an expression like $3a + 4$, it signifies that for any given value of $a$, you multiply that value by $3$ and then add $4$ to the result. For instance, if $a$ is $2$, then $3a + 4$ can be calculated as follows:

If $a$ were $5$, the calculation for $3a + 4$ would be:

Understanding variables is essential in algebra, as they enable you to formulate general expressions and equations that can be solved for various values. This flexibility is what makes algebra a powerful tool in mathematics. By learning to manipulate and solve expressions involving variables, you can address a wide array of problems, ranging from simple equations to more complex functions.