Помогите пожалуйста.

Решите задачу:

$12) \frac{25 {x}^{2} - {y}^{2} }{10xy} \div ( \frac{1}{y} - \frac{1}{5x} ) = \frac{(5x - y)(5x + y)}{10xy} \times \frac{5xy}{5x - y} = \frac{5x + y}{2} = \frac{5 \times 3 \frac{2}{17} + 2 \frac{7}{17} }{2} = \frac{5 \times \frac{53}{17} + \frac{41}{17} }{2} = \frac{265 + 41}{34} = \frac{306}{34} = 9$
$13)q = {i}^{2} rt \\ q = 1960 \\ i = 7 \\ r = 8 \\ 1960 = {7}^{2} \times 8 \times t \\ 1960 = 392t \\ t = \frac{1960}{392} = 5c$