Question

3-x/4+x=8/48

Answer 1

Supposing you mean (3-x)/(4+x)=8/48
Cross multiply:
48 (3-x) = 8 (4+x)
Expand:
144 - 48 x = 32 + 8x
Combine like terms:
56x = 112
Divide by 56:
x = 2

[I don't see any chemistry in this problem.]