Question

How to do this! Completely lost!!

Answer 1

2 ways, subsitute or eliminate
subsitute is slightly easier

add 7y to both sides in first equation
x=7y-30
sub 7y-30 for x in 2nd equaiton
9y-7(7y-30)=90
9y-49y+210=90
-40y+210=90
minus 210 both sides
-40y=-120
divide both sides by -40
y=3

sub back

x=7y-30
x=7(3)-30
x=21-30
x=-9


(x,y)
(-9,3) is solution

A. (-9,3)

Answer 2

You can actually solve this, if you know what either x or y value is.
x - 7y = -30 (1)
9y - 7x = 90 (2)
1. From equation (1), you can rearrange the equation so you make 'x' the subject.
x - 7y = -30
x = -30 + 7y
2. Now you know what 'x' is, you can substitute it into equation (2).
9y - 7(-30 + 7y) = 90
3. Solve for y.
9y + 210 - 49y = 90
-40y = -120
y = 3
3. After you solve for y, substitute your y value (3) back into equation (1) to find out what x is.
x - 7y = -30
x - 7(3) = -30
x - 21 = -30
x = -9
Therefore (recalculated), x= -9, y=3