Remember that the the sum of the internal angles of a triangle is equal to 180 degrees. This is true for all triangles.
So, we can set up an equation like this,
54 + 38 + X = 180 [where X is the number of degrees of the third angle]
Now solve for X by subtracting 54 and 38 from both sides,
X = 88
Therefore, the third angle is 88 degrees.