Given,

A = x (y′ + z′)

= xy′ + xz′

To express as a complete sum-of-products, expand using all variable combinations:

= xy′(z + z′) + xz′(y + y′)

= xy′z + xy′z′ + xyz′ + xy′z′

After simplification, duplicate terms are removed:

A = xyz′ + xy′z + xy′z′