Did you mean y - 2 = (-3/4)(x + 5) or y - 2 = -3 / (4(x - 5)) ?
The first one's pretty easy (see below), but the slope for the second one isn't just a number; it's an equation and you need this thing called "derivatives" that you won't encounter for 2-3 more years.
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y - 2 = (-3/4)(x + 5)
When you get slope intercept form (y = mx + b), m is the slope. It's whatever number sits in front of the x.
Moving the equation for this problem around will get us slope intercept form, but we can already see the slope sitting in front of the x. It should be -3/4, but let's make sure.
y - 2 = (-3/4)(x + 5)
y - 2 = (-3/4)x - 15/4
y = (-3/4)x - 15/4 + 2
y = (-3/4)x - 15/4 + 8/4
y = (-3/4)x - 7/4
The slope is -3/4