1. **State the problem:** We need to find the value of $c$ given the equation $c + 42 = 90$. 2. **Formula and rule:** To isolate $c$, subtract 42 from both sides of the equation. This uses the subtraction property of equality which states that if you subtract the same number from both sides, the equality remains true. 3. **Perform the subtraction:** $$c + 42 - 42 = 90 - 42$$ 4. **Simplify both sides:** $$c + \cancel{42} - \cancel{42} = 90 - 42$$ $$c = 48$$ 5. **Interpretation:** The value of $c$ is 48 degrees. This makes sense because the two angles $c$ and 42° add up to 90°, which is a right angle. **Final answer:** $$c = 48$$