Explanation:
This question gives us a point on a line and the slope of that line. So, the easiest way to answer this question is to use point-slope form, which is written as:
y-y₁ = m(x-x₁)
So, first of all, we are trying to find y (y, not y₁ which is the y-coordinate of the point we were given). We can go ahead and add y₁ to both sides to get:
y = m(x-x₁) + y₁
We can now simplify the equation to:
y = m·x - m·x₁ + y₁
Now we just input the given information.
m = 5 (slope)
x = 21
x₁ = 5
y₁ = 6
Now we have all the information we need and we can solve.
y = 5·21 - 5·5 + 6
y = 105 - 25 + 6
y = 86
So y, when x = 21, is equal to 86.