Go to PoolMath and see how much Muriatic Acid it will take to lower pH to 7.2 - add the acid slowly and one quart at a time and retest every half hour. It is very strong. Handle with care. You can get MA at Lowes in the paint section.
Next add Stabilizer in a sock hanging in front of the return with pump on to raise CYA to 30-40 (check PoolMath for amount. Again, better to put in less and slowly than to overshoot. There is no way to lower CYA other than replacing the water, so be careful.)
When your pH is 7.2 and CYA 30-40, then check the CYA/Chlorine chart and maintain above your Shock level according to the SLAM procedure.
but you have to put your info in PoolMath first and get those numbers up before you start to SLAM
Also, go to the Pool School section and read up on the process. It sounds more complicated than it is. Hardest part is patience - can take a couple weeks
One more thing, are you sure your pool is 45,000 gallons? That is a huge pool and the size makes a big difference when it comes to chemicals. You do not want to add too much, as I said above, some are impossible to remove. Just check to make sure you know exactly how many gallons your pool holds.