How do I solve solve this? find the gradient of y + 7x = 8?

Method 1: Slope-intercept form of straight line

y + 7x = 8
Rearrange as y = -7x + 8
Refer to the slope-intercept form of straight line: y = mx + C where m is the gradient.
Hence, gradient = -7


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Method 2: Differentiation

y + 7x = 8
(d/dx)(y + 7x) = (d/dx)8
(dy/dx) + 7 = 0
Slope, dy/dx = -7


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Method 3: Using arbitrary two points on the straight line

y + 7x = 8
When y = 1: x = 1
When x = 0: y = 8
Hence, (1, 1) and (0, 8) are two

Slope of the straight line (1 - 8)/(1 - 0) = -7

Really?
Gradient = -7

y+7x=8=>
y=-7x+8=>
gradient=-7

y = 8 -7x. dy/dx = {d/dx}(8-7x) = -7.

Put the equation in y = mx + b form.
y + 7x = 8
y = -7x + 8
gradient is -7

There is nothing to "solve", you just have to apply your definition of a gradient.

In its standard definition, a gradient applies to a function of several variables, and your question would make no sense.
However in English the word gradient is somewhere used to actually mean the slope, i.e. dy/dx. In this case it is obviously -7: since the equation you're given is true for any x, you can take its derivative with x, which gives you dy/dx+7=0

should have something else ?