If 3.50 mol of A is mixed with 0.750 mol of B and the final pressure of the mixture is 3.25 atm, what are the partial pressures of A and B
You don't say but I assume A and B are gases.
total mols = mols A + mols B
X(A) = mol fraction A = mols A/total mols.
XB = mols fraction B = mols B/total mols
pA = XA*Ptotal
pB = XB*Ptotal