According the the International Game Fish Association formula (LxGxG divided by 800), the fish would be 12.4 pounds. However the problem with that formula is the calculation was derived from studying albacore tuna. The correlation coefficient compared to actual weight is .88.
Now according to Lewis Lum a mathamatics professor at the University of Portland, the true calcuation for steelhead should be LxLxGx0.0007. That would make this fish at just under fifteen pounds. The correlation coefficient is .92.
There is an article on this by Pat Hoglund in the Spring 2006 edition of Salmon & Steelhead Journal.