A beаm hаs dimensiоns оf b = 18 in., h = 28 in., аnd d = 25.5 in. and is reinfоrced with 4 No. 7 bars. The concrete strength is 3,100 psi, and the yield strength of the reinforcement is 60,000 psi. Assuming the reinforcement has yielded, determine the depth to the neutral axis, c.