It is often rainy at/in autumn

It is often rainy IN autumn.

Explanation:

IN:

Streets

Specific places

Cities

Countries

Continents

Parts of a country / region / city

Number of weeks / months / years

Parts of the day

Months

Seasons

Years

Decades

Centuries

ON:

Weekdays

Special days with the word "day"

Specific dates

AT:

Specific places in towns or cities

Hours

Parties and festivals without the word "day"

substitute y = 2x - 4 to x + 2y = 12:

x + 2(2x - 4) = 12     use distributive property

x + (2)(2x) + () = 12

x + 4x - 8 = 12     add 8 to both sides

5x = 20     divide both sides by 5

x = 4

put the value of x to the first equation:

y = 2(4) - 4

y = 8 - 4

y = 4