supplying an expression without any keyword first tries to solve the
expression by default but can be made more effective by using it explicitly.
the expressions is assumed to be equal to zero.
In case of numerous variables, the variable to be solved for may be specified,
else one will be chosen automatically, usually x if present. See structure below.
For example, exp(x)+1, exp(x)+1, x and solve(exp(x) + 1, x) all produce the same result
To solve simultaneous equations, first make each equations to be equal to 0, then supply as show below.
solve([eq1,eq2...], [var1,var2...]). For example: solve([x + 5*y - 2, -3*x + 6*y - 15], [x, y])