4. Let the demand and supply functions for q units of a certain item be p = D(q) = 61e61293daed8.jpgand p = S(q) = 61e612a8db809.jpg, q ＞ 1, where p is in dollars. Its cost function is C(q) = 61e612b409ef3.jpg . Find (a) equilibrium quantity； (b) break-even quantity.