Пошаговое объяснение:
5:13=a:39
13 * a = 5 * 39
13a = 195
a = 195 : 13
a = 15
2(2+x)+3(4x-1)=10x-7
4 + 2x + 12x - 3 = 10x - 7
1 + 14x = 10x - 7
14x - 10x = -7 - 1
4x = -8
x = -8 : 4
x = -2
