[The narrator reads out the onscreen text.]

NARRATOR: Question three. Part a. If 2x plus 3y equals 6, find x when y equals 4.

Part b. If y equals 5x minus 10, find x when y equals 20.

Part a. Here we have 2x plus 3y is equal to 6. And we're asked to find the value of x when y is equal to 4. So we will substitute 4 in place of y, giving 2x plus 3 times 4 equal to 6. So 2x plus 12 equals 6. And now we have an equation that we must solve for x. So if we subtract 12 from both sides of the equation, we'll be left with 2x on the left-hand side and negative 6 on the right-hand side. And if we divide both sides of the equation by 2, we'll be left with x on the left-hand side and negative 3 on the right-hand side. So when y is equal to 4, x is equal to negative 3.

Part b. Here we have the relationship y equals 5x minus 10, and we wish to find x when y is equal to 20. So we will substitute 20 in place of y.

[20 = 5x - 10]

NARRATOR: And we now have an equation that we might solve for x. So adding 10 to both sides gives 30 equal to 5x. And then dividing both sides of the equation by 5 gives 6 equal to x. So when y is equal to 20, x is equal to 6.