What is the solution of the equation x2 + 10x + 75 = 0?

There isn't one unless you know what complex numbers are. If you do then use the quadratic formula to get the answer.
x = [- b +/- sqrt( b^2 - 4ac ) ] / 2a
a = 1
b = 10
c = 75

x = [ -10 +/- sqrt(-200)]/2
x = [ -10 +/- 10 sqrt(2) / 2
x =  - 5 + / - 5 sqrt(2) and that's your answer.