3x+4 is the average of 2x+2 and 8x-14 because it's exactly between the two on a trapezium. therefore:
3x+4=(2x+2 + 8x-14) / 2
↓
3x+4=(10x-12) / 2
↓
3x+4=5x-6
↓ add 6 to both sides
3x+10=5x
↓ take away 3x from both sides
10=2x
↓ divide both sides by 2
5=x