The actual beam deflection at B (V_B) is known — since B is a roller support, V_B = 0. Add the Case 1 and Case 2 expressions for the beam deflection at B and set the sum equal to zero. Solve for the unknown reaction force B_y.

Since EI appears in the denominator of each term, it can be cancelled out. In executing this calculation, you must always take care to ensure that the units are consistent.