If I knew that the area of my yard is 30 sq ft and I had 34 ft of fencing, how would I solve this problem? I can't figure this one out. Please help!

2(l+w)=34 ft of fencing, where l= length and w= width of this rectangle
l*w=30

2l+2w=34
2l=34-2w
l=17-w

sub l into l*w=30
(17-w)*w=30
-(w square) + 17w=30
using completing the square
-(w squared -17w + ((17)square/(2) square)-(17square /2 square)=30
-(w- (17/2)square)square + 72.25=30
-(w- (17/2)square)square=-42.25
turns both side positive
(w- 72.25)square=42.25
squareroot

w-72.25=+- sqrt 42.25
w1=+6.5+72.25 w1=78.75
w2=-6.5+72.25 w2=65.75

edit* hopes this help, if don't get it, go on Google and search as to how to solve quadratic equations.