Use Symbolic Math Toolbox. Define your constants symbolically.
% Define the constants
B_2 = sym(0.18525);
H = sym('3000175332800');
D = sym(0.9999932);
z = sym(2.8) * 10^10;
I made no modifications below this line besides removing the blank line at the end of the code block.
% Define the expression for R
R = (log(z))^(3/5) / (log(log(z)))^(1/5);
% Display R
disp('R:');
disp(R);
RR = double(R);
disp('RR:');
disp(RR);
% Define the expression for b
b = (log(H/(2*pi)))^2 * (exp(-0.5 * z) - 1) * z^((6 * D - 15) / 5) * exp(B_2 * (8 * D - 5) * R / 3);
% Evaluate b
bb = double(b);
% Display b
disp('b:');
disp(b);
disp('bb:');
disp(bb);