This is a two part question I'm so confused.

A) ABC Company decided to trade in computer #48 for a new computer #50 and paid $20,000 cash. Computer #50 will be used for 3 years with the salavage/trade-in value of $15,000. Invoice for exchange showed the following:

Price of new computer #50 $90,000
Trade-in allowance granted on old computer #48 $70,000
Total of purchase price paid in cash $20,000

B) Please record straight line depreciation on computer # 50.

I'm sorry I only see the question for part B, which is:
New Computer : $90,000
Minus Salvage : $15,000
Equals : $75,000

Depreciatiion expense for one year is: $75,000/3 = $25,000

Hope this helps