I can't figure out how to do this problem. I tried using the quadratic formula, but ended up getting a negative under the square root. Any help would be greatly appreciated!

The target weight w for an adult man of medium build is:

w(h)=0.0728h^2 - 6.986h + 289 where h is in inches and w is in pounds. If a man of medium build has achieved his target weight of 170 lb, how tall is he?