We first write out the whole equation mathematically.
-3 - 5(3x + 7) = 0
we expand the brackets first according to the BODMAS rule of mathematics.
-3 -15x - 35 = 0
we then begin to put all the known values on one side and the unknown value x on the other side.
-15x - 35 = 3
-15x = 3 + 35
-15x = 38
we divide both sides of the equation by -15
-15x / -15 = 38 / -15
x = 38 / -15
x = -2.53