There a number of methods in solving a system of equations. These methods are the substitution method, solving by graphing, gaussian elimination and by elimination method. For this case, we use the substitution method. To do this, we write one of the equation in a way where one variable is left on one side. We do as follows:
−0.9x+0.5y=3.9
0.5y = 3.9 + 0.9x
y = 3.9/0.5 + 0.9x/0.5
y = 7.8 + 1.8x
Then, we subsitute this equation to variable y in the second equation.
0.4x+1.5(7.8 + 1.8x)=−6.9
Then, we solve for x
0.4x+1.5(7.8 + 1.8x)=−6.9
0.4x+11.7 + 2.7x=−6.9
3.1x = -18.6
x = -6
We substitute this value of x, to the first equation to solve for y
y = 7.8 + 1.8x
y = 7.8 + 1.8(-6)
y = -3