21х+5х=312

To solve the equation \( 21x + 5x = 312 \), first combine like terms.

\[ 21x + 5x = 26x \]

So, we have:

\[ 26x = 312 \]

Next, divide both sides by 26 to isolate \( x \):

\[ x = \frac{312}{26} \]

Calculating this gives:

\[ x = 12 \]

Thus, the solution is:

\[ x = 12 \]