Figured it out!
% Solve for life using SWT equations for a, b, and c.
Sa = 220; % Stress amplitude in MPa given in problem
Sm = [0 110 -110]; % Given stress as a matlab vector (MPa)
Sf = 900; % MPa
Smax = Sm + Sa; % Calculate maximum stress
b = -0.102; % Correction factor for material
N = (nthroot((sqrt(Smax*Sa)/Sf),b))/2; % Lifefor mean stresses using SWT equation