Question 495770
You did a great job with the expansion, and your way will work if you use the quadratic formula ({{{x = (-b +- sqrt( b^2-4*a*c ))/(2*a) }}}) using a=49, b=84, and c=72.


However, another approach to take is to use the fact that the polynomial is already written as a perfect square.  If you take the square root of both sides, you'll have {{{ sqrt((7x+6)^2)= "+-" sqrt(-36) }}}, or {{{7x+6="+-"6i}}}  (sorry about the awkwardness with the positive/negatives...just remember that when you take the square root on both sides, you could get either a positive or negative).  To solve, subtract 6 from both sides ({{{7x=-6 +- 6i}}}) then divide everything by 7: {{{x=-6/7+-expr(6/7)i}}}; broken up, you two answers are {{{x=-6/7+expr(6/7)i}}} and {{{x=-6/7-expr(6/7)i}}}