First term and length

In an arithmetic progression the second term is $7\frac{3}{4}$, the thirty‑first term is $\frac{1}{2}$, and the last term is $-6\frac{1}{2}$. Find the first term and the total number of terms.

H. S. Hall & S. R. Knight, Higher Algebra (1891) · Ch. IV. Arithmetical Progression, Examples IV.b, No. 4