1.4x - 0.4(t - 3.1) = 5.8
1.4x - 0.4t + 1.24 = 5.8 distributed 0.4 on left side
1.4x - 0.4t = 3.56 subtracted 1.24 from both sides
This is as far as you can go if they are different variables
************************************************************************
If you typed it wrong and they are supposed to be the same variable, then:
1.4x - 0.4x = 3.56
1.0x = 3.56
x = 3.56