When you put the segment HI on one side of GH you form GI, then, the length of GI equals the sum of the lengths of GH and HI, with this information, we can formulate the following expression:
GH + HI = GI
45 + HI = 61
From this equation, we can solve for HI, like this:
45 - 45 + HI = 61 - 45
HI = 16
Then, HI equals 16