A wave has more to it than its height. Two waves can have the same amplitude and frequency and still combine very differently. The missing ingredient is their relative phase: where one wave sits within its cycle compared with the other.
Predict before you calculate.
Draw two equal sine waves with their peaks aligned. Add their heights at a few positions. Then shift one wave by half a cycle and repeat. In the second drawing, every positive displacement meets an equal negative displacement.
Make the model explicit.
This picture assumes equal amplitudes, identical frequencies, and the same propagation direction. Stating those assumptions is part of the explanation. Real experiments can include losses, unequal amplitudes, and additional modes.
Test your prediction ↗