Question 1172888
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<pre>

Introduce new variable  u = 5x.

Then the equation takes the form


    u^2 + 6u + 8 = 0.


Factor it mentally in 1 second


    (u + 4)*(u + 2) = 0


The roots are  u= -4  and  u= -2.


Hence, the roots of the original equations are


    a)  5x = -4,  x = {{{-4/5}}},

and

    b)  5x = -2,  x = {{{-2/5}}}.
</pre>

Solved // mentally.