The number of integral values of k for which the equation 7 cos x +5 sin x =2k +1 has a solution is
−√a2+b2≤acosθ+bsinθ≤√a2+b2⇒−√74≤7cosx+5sinx≤√74⇒−√74≤2k+1≤−√74⇒−8≤2k+1≤8⇒−4.5≤k≤3.5
Considering only intergral values, which means k can take eight integral values.