Разберем каждый пункт пошагово:
1) $$9.88 : a = 3.8$$
$$a = 9.88 / 3.8$$
$$a \approx 2.6$$
$$3.8 - b = 1.74$$
$$b = 3.8 - 1.74$$
$$b = 2.06$$
$$1.74 \cdot c = 6.09$$
$$c = 6.09 / 1.74$$
$$c = 3.5$$
Ответ: $$a \approx 2.6$$, $$b = 2.06$$, $$c = 3.5$$
2) $$6.2 \cdot x = 17.36$$
$$x = 17.36 / 6.2$$
$$x = 2.8$$
$$17.36 + y = 20.1$$
$$y = 20.1 - 17.36$$
$$y = 2.74$$
$$20.1 : z = 1.5$$
$$z = 20.1 / 1.5$$
$$z = 13.4$$
Ответ: $$x = 2.8$$, $$y = 2.74$$, $$z = 13.4$$