In order to find the gradient of the line y = 3x - 7, take a look at the number that appears before 'x'; this number is 3.
Henceforth, the gradient is 3.
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Bonus Question:
What is the y-intercept of the line y = 3x - 7
In order to find the y-intercept of the line y = 3x - 7, take a look at the constant term; the term is -7.
Henceforth, the y-intercept is -7.
KEY INFO
This only works for equations that are formatted like y = mx + b, which is slope-intercept (m is the gradient/slope and b is the y-intercept). There are other forms of lines' equations, but the slope is different.