Final answer:
It appears that there is information missing from the question as it stands, making it impossible to definitively calculate the length of line AC. We understand that we have a right triangle, with AB being 51 units long and angle A being 90°, but without further context or information relating line AB and line AC, the value of X and hence the length of line AC remain undetermined.
Step-by-step explanation:
To calculate the length of line AC when line AB is 51, line AC is 144 - X, and angle A is 90°, it is essential to identify the type of triangle formed by these lines. Considering angle A is a right angle, we are dealing with a right triangle. Therefore, if AB represents one leg and AC the other leg, we can use the Pythagorean theorem to find the value of X, which would then give the length of AC.
However, we are given AC as an expression (144 - X) rather than a fixed length, implying there is additional information or context missing from the question itself that would relate AB and AC. As this information is not provided in the question or reference texts, we are unable to compute a specific value for the length of AC or the variable X.