3x+1 = 100

log10(3x+1) = log10(100)

(x+1)10log(3) = log10(100)

 = 

 = 

x+1 = 

x+1 = 4.191806549

  x = 3.191806549

Edwin

The above tutor thought you meant

3x + 1 = 100

because you didn't put the exponent in parentheses, which
you should have done.  However I and the first tutor above
knew that by the time you get to logarithms, no teacher would
bother giving you a problem that would waste time to test you
on such a simple basic thing like whether you can subtract 1  
from both sides.  That's too elementary a thing to be testing 
someone already studying logarithms. So I assumed you meant

3x+1 = 100

You can also do it using natural logs instead of logs base 10

3x+1 = 100

ln(3x+1) = ln(100)

(x+1)ln(3) = ln(100)

 = 

 = 

x+1 = 

x+1 = 4.19180655

  x = 3.19180655

Edwin