Ineed with all of them and if you can't with all of them just answer one of them . look at the picture

All you have to do is substitute the y value from the 1st equation into the second equation and solve...

a) y= 2-x
5x + 4y = 5

Substitute (2-x) into the second equation anywhere there is a y...

5x + 4y = 5
5x + 4(2-x) = 5

Now solve

5x + 8 - 4x = 5
5x - 4x + 8 = 5
x + 8 = 5
x = -3

Now that you have a solution for x, substitute -3 into either of the original equations anywhere there is an x then solve for y...

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

You solved for x and got -3 and solved for y and got 5, so your solution set is
(-3, 5).

Now check it by substituting both numbers into one of the original equations and you should have a true statement if it is correct...

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

True statement... it checks!

note* during the check, if the equation would have worked out to something like 2 = 5, then that is a false statement therefore the solution set would be wrong and you'd have to go back and find the mistake.

Element 17 is chlorine it is a group 7 element(halogen) and belongs to period 3 of the periodic table