A+B =10,500 ...........................................(1)
B*1.5 = A ..................................................(2)
Replace 'A' in (1) by the value in (2)
(B*1.5) + B =10,500
Factoring out 'B' _ you can write:
B * (1.5 + 1) = 10500
B * 2.5 = 10500
Continue....