Start by doing the calculation on the LHS using x=4.5. If the answer is less than 80 you know x>4.5. If it's greater than 80 then x<4.5.
You can then try different values of x to narrow down the range of values for x to two values 1dp apart (eg x is between 4.2 and 4.3).
Once you have calculated this use a value of x half way between these values (in my example, 4.25) to determine if x is bigger or smaller than the midway point. This will help you determine the answer to 1dp (so if x is between 4.2 and 4.25 the answer is 4.2, but if it's between 4.25 and 4.3 the answer is 4.3).
Sorry for my clumsy explanation, hope this makes sense!