book Maths Puzzle Numerical Ability

my questions is :
find the value of r so that the line through (8,r) and (4,5) has a slope of -4.

a.11
b.-11
c.4
d.-4
thanks for your help!

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book Other Question

The equations s1=18+0.4m and s2=11.2+0.54m give the lengths of two different springs in centimeters, s1 and s2, as mass amounts in grams, m, are separately added to each.
a. When
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-find the value of x
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