Algebra: solving systems with substitution and elimination, ex:
{y=2x
3x+y=5

subsitution means subsituteing something for another and getting the answer
the way to do this is to get an easy equation and make it equal to only one placeholder so
y=2x
so you can subsitute y for 2x anywhere you see a y
3x+y=5
subsitutute
3x+2x=5
5x=5
divide both sides by 5
x=1
subsitute
y=2(1)
y=2

elimination is when you elimintate one of the terms by adding the equations together and canceling som of them out
so y=2x
subtract y from both sides
0=2x-y or 2x-y=0

add this to the second equation
3x+2x+y-y=0+5
5x=5
divide both sides by 5
x=1
subsitute
y=2(1)
y=2

x=1
y=2

I'll try my best to answer this question to the best of my abilities. 

3x+y=5
3x+(2x)=5
5x=5
x=1