a, \(A=\left\{x\in N;x=7.q +3;q\in N;x\le150\right\}\) Inferred

\(A=\left\{3;10;17;24;.........;143;150\right\}\)

b, easy to see numbers \(3;10;17;24;....;143;150\)is the sequence of numbers plus  \(u_1=3;d=7.\)

The term number of the sequence \(\left(1\right)\)was:

\(n=\dfrac{u_n-u_1}{d}+1=\dfrac{150-3}{7}+1=22\) (term)

Sum the numbers of the sequence \(\left(1\right)\)was:

\(S_n=\dfrac{\left(u_n+u_1\right)\times n}{2}=\dfrac{\left(3+150\right)\times22}{2}=1683.\)