Before isolating s we need to simplify the equation using order of operations.
.7(3s+4)-1.1s = 7.9 -------> First distribute the .7
2.1s + 2.8 - 1.1s = 7.9 ------> Now combine like terms
s + 2.8 = 7.9 ------> Now to isolate s by subtracting 2.8 from both sides!
s = 7.9 - 2.8
s = 5.1