You have to provide the correct values for the constants.
This will provide you with the numeric result:
syms d_i er Re x
d_i = sym(3); % Insert Correct Values
er = sym(5); % Insert Correct Values
Re = sym(1000); % Insert Correct Values
Eq = x+4*log10(er/(3.7*d_i)+1.4125*x/Re) == 0;
x_sol = solve(Eq, x, 'IgnoreAnalyticConstraints',1);
x_val = double(x_sol) % Desired Numeric Result
EDIT — You could do this numerically with fzero if you know the approximate value of the desired root:
d_i = 3; % Insert Correct Value
er = 5; % Insert Correct Value
Re = 1000; % Insert Correct Value
fcn = @(x) x+4*log10(er/(3.7*d_i)+1.4125*x/Re); % Anonymous Function
x_val = fzero(fcn, 10.8); % Solution
