Given the formula a = 2b + 3, we can test to see whether the solution a = 10, b = 4 works.
We substitute b = 4 into the formula to see whether it gives a = 10.
a = 2b + 3
a = 2 x b + 3
a = 2 x 4 + 3
a = 8 + 3
a = 11
So a = 10, b = 4 does not fit.