Subtract 4 from both sides. You'll get 8x = 28. Solving for x, x = 28/8 = 7/2
29% = 0.29
Multiply 1.89 with 0.29
1.89 x 0.29 = 0.5481
Note that it is a markdown, so subtract 0.55 (rounded) with 1.89
1.89 - 0.55 = 1.34
The spark plug now costs ~ $1.34
hope this helps
