Two angles are supplementary if the sum of their measures equals 180 degrees.
Given:
- \( m∠ACB = x° \)
- \( m∠BCD = 26° \)
Since these two angles are supplementary, we can write the equation:
\[ x + 26 = 180 \]
To find \( x \), subtract 26 from both sides:
\[ x = 180 - 26 \] \[ x = 154 \]
Therefore, the measure of \( ∠ACB \) is \( 154° \).