If two angles are supplementary, their measures add up to 180 degrees. Given:
- \( m∠ACB = x° \)
- \( m∠BCD = 26° \)
We can set up the equation:
\[ x + 26 = 180 \]
To find \( x \), we can subtract 26 from both sides:
\[ x = 180 - 26 \]
Calculating that gives:
\[ x = 154 \]
Therefore, the measure of \( ∠ACB \) is:
\[ m∠ACB = 154° \]