Sum two columns lying in two different table based on a condition

Question

Table 1

Platform	Sales	Cost
A	100	50
B	200	100
B	300	100
D	50	10

 

Table 2

Channels	Result
A	50
B	100
B	50
C	200

 

I wanted to create a calculated field for platform B, that adds Sales # for platform B from table 1 and add the Results# for Channels B from Table 2. So, the calculated field should give me Platform B total = 200 + 300 + 100 +50 = 650. Basically in Channels and Platforms are same. But I need this calculated field specifically for Platform B. Does it make sense?  Any help you can provide that will be awesome!

Thank you in advance.