7х-3х=21

To solve the equation \( 7x - 3x = 21 \), follow these steps:

1. Combine like terms on the left side of the equation: \[ (7x - 3x) = 4x \] So, the equation becomes: \[ 4x = 21 \]

2. Next, to isolate \( x \), divide both sides of the equation by 4: \[ x = \frac{21}{4} \]

3. Simplify \( \frac{21}{4} \) if necessary. In this case, it remains as is.

Therefore, the solution is: \[ x = \frac{21}{4} \quad \text{or} \quad x = 5.25 \]