Any quadratic problem will always have two solutions. Therefore since, we only have one solution here, it is wrong. The mistake was incurred in step 2;
The problem states that ABC and DEF are similar. Therefore, dsue to similar triangles, we have 7x/7=4/x, so x^2=4. Therefore, x=2 (because x cannot be negative). Therefore, AB = 7x = 7*2=14